Farm animals head off for crop (4)
I believe the answer is:
oats
'for crop' is the definition.
(type of plant)
'farm animals head off' is the wordplay.
'farm animals' becomes 'goats' (goats might be kept on a farm).
'head off' means to remove the first letter.
'goats' with its first letter taken away is 'OATS'.
(Other definitions for oats that I've seen before include "Young folk might sow wild ones" , "Cereal grass and seeds" , "Cereal plants" , "Porridge ingredients" , "Sow wild ones in disreputable youth" .)