Farm animal requires long rearing (7)
I believe the answer is:
rampant
'rearing' is the definition.
('rampant' can be a synonym of 'rearing')
'farm animal requires long' is the wordplay.
'farm animal' becomes 'ram' (I've seen this before).
'requires' is a charade indicator (letters next to each other).
'long' becomes 'pant' (I've seen this before).
'ram'+'pant'='RAMPANT'
(Other definitions for rampant that I've seen before include "Out of control" , "Uncontrolled" , "Spreading like wildfire" , "running wild" , "Unchecked - standing up on hind legs" .)