Fine old animal heading west (4)
I believe the answer is:
okay
'fine' is the definition.
(I've seen this before)
'old animal heading west' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'animal' becomes 'yak' (I have seen 'Tibetan animal ' mean 'yak' so perhaps 'animal' could also mean 'yak').
'heading west' says the letters should be written backwards.
'yak' written backwards gives 'kay'.
'o'+'kay'='OKAY'
(Other definitions for okay that I've seen before include "Leave" , "Yes; satisfactory" , "Agree" , "Approve, endorse" , "All right; sanction" .)