From Style in Guide part of ABC of Plain Words by Sir E Gowers (1951)

Fowler's definition of a cliché is:

A French name for such hackneyed phrases as, not being the simple or natural way of expressing what is to be expressed, have served when first used as real improvements on that in some particular context, but have acquired an unfortunate popularity and come into general use even when they are not more but less suitable to the context than plain speech.

This definition may be rounded off by Eric Partridge's comment:

They range from fly-blown phrases (explore every avenue) through sobriquets that have lost all point and freshness (the Iron Duke) to quotations that have become debased currency (cups that cheer but not inebriate), metaphors that are now pointless, and formulas that have become mere counters (far be it from me to . . .).

A cliché then is by definition a bad thing, not to be employed by self-respecting writers. Judged by this test, some expressions are unquestionably and in all circumstances clichés. This is true in particular of verbose and facetious ways of saying simple things (conspicuous by its absence, tender mercies, durance vile) and of phrases so threadbare that they cannot escape the suspicion of being used automatically (leave no stone unturned, acid test, psychological moment, leave severely alone). But a vast number of other expressions may or may not be clichés. It depends on whether they are used unthinkingly as reach-me-downs or deliberately chosen as the best means of saying what the writer wants to say. Eric Partridge's Dictionary of Clichés contains some thousands of entries. But, as he says in his preface, what is a cliché is partly a matter of opinion. It is also a matter of occasion. Many of those in his dictionary may or may not be clichés; it depends on how they are used. Examples are: to cross the Rubicon, to cry over spilt milk, a work of supererogation and to break the ice. Such phrases as these may be the fittest way of expressing a writer's meaning. If you choose one for that reason, and not because you think it fine, or because it is the first thing that came into your head, you need not be afraid of being called a cliché-monger.