Farm animal, one that's agitated bird (6)
I believe the answer is:
pigeon
'bird' is the definition.
(type of bird)
'farm animal one that's agitated' is the wordplay.
'farm animal' becomes 'pig' (type of farm animal).
'that's agitated' indicates anagramming the letters.
'one' with letters rearranged gives 'eon'.
'pig'+'eon'='PIGEON'
(Other definitions for pigeon that I've seen before include "Bird of towns and woods" , "Short-legged bird" , "gull" , "Common bird, wild and domesticated" , "Seed-eating bird" .)