Bath Oliver’s last in box (4)
I believe the answer is:
spar
'box' is the definition.
(sparring is a kind of boxing)
'bath oliver's last' is the wordplay.
'bath' becomes 'spa' (I've seen this in another clue).
'last' indicates one should take the final letters.
The final letter of 'oliver' is 'r'.
'spa'+'r'='SPAR'
'in' is the link.
(Other definitions for spar that I've seen before include "bar" , "Box - girder" , "Argue" , "Practice boxing with wooden beam" , "Engage in argument" .)