Saw metaphor is meaningless to some extent (8)
I believe the answer is:
aphorism
'saw' is the definition.
(I've seen this before)
'metaphor is meaningless to some extent' is the wordplay.
'to some extent' says the answer is hidden in the clue.
'APHORISM' is hidden within 'metaphor is meaningless'.
(Other definitions for aphorism that I've seen before include "Pithy observation which contains a general truth" , "Terse saying" , "Saw" , "Maxim, short wise saying" , "A short, pithy, instructive saying" .)