Fellow keeping oxen at first for beef (4)
I believe the answer is:
moan
'beef' is the definition.
(I've seen this before)
'fellow keeping oxen at first' is the wordplay.
'fellow' becomes 'man' (I've seen this before).
'keeping' is an insertion indicator.
'at first' says to take the initial letters.
The first letter of 'oxen' is 'o'.
'man' enclosing 'o' is 'MOAN'.
'for' is the link.
(Other definitions for moan that I've seen before include "Complain; rhymes with money lent" , "Low sound of pain" , "Whinge" , "Keen" , "Express pain" .)