Meat joint and large onion used occasionally (4)
I believe the answer is:
loin
'meat joint' is the definition.
(I've seen this before)
'and large onion used occasionally' is the wordplay.
I cannot quite see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
'large' could be 'l' (eg in clothes sizes) and 'l' is found in the answer.
The remaining letters 'oi' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for loin that I've seen before include "Part of a carcase" , "Cut of meat from between the ribs and rump" , "Choice cut of meat" , "Cut of meat from between rib and haunch" , "Part of body between ribs and hip-bone" .)