Cheese a fly would not be on for long (4)
I believe the answer is:
brie
'cheese' is the definition.
(I've seen this before)
'be on for long' is the wordplay.
'on' is an insertion indicator (as in clothing \'on\' a person).
'for long' becomes 'ri' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'be' going around 'ri' is 'BRIE'.
'a fly would not' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for brie that I've seen before include "dairy product" , "French 8 across" , "French cream cheese" , "French produce" , "cheesy stuff" .)