Old ox rising fine (4)
I believe the answer is:
okay
'fine' is the definition.
(I've seen this before)
'old ox rising' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ox' becomes 'yak' (yak is a kind of ox).
'rising' says the letters should be written backwards (going up in a down clue).
'yak' backwards is 'kay'.
'o'+'kay'='OKAY'
(Other definitions for okay that I've seen before include "Approve, endorse" , "Correct" , "Give approval to" , "All right; sanction" , "Agree" .)