Saw farm animal trapped by rearing horse (7)
I believe the answer is:
epigram
'saw' is the definition.
(thesaurus)
'farm animal trapped by rearing horse' is the wordplay.
'farm animal' becomes 'pig' (I've seen this before).
'trapped by' is an insertion indicator.
'rearing' says the letters should be written in reverse (as in a horse rearing up).
'horse' becomes 'mare' (mare is a kind of horse).
'mare' reversed gives 'eram'.
'pig' inserted into 'eram' is 'EPIGRAM'.
(Other definitions for epigram that I've seen before include "Concise witty saying" , "A clever witty saying" , "Bon mot" , "witty comment" , "it might be natural to wag" .)