Produce attack to seal game (5)
I believe the answer is:
fruit
'produce' is the definition.
(I've seen this in another clue)
'attack to seal game' is the wordplay.
'attack' becomes 'fit' (fit is a kind of attack).
'to seal' indicates putting letters inside.
'game' becomes 'RU' (rugby union).
'fit' going around 'ru' is 'FRUIT'.
(Other definitions for fruit that I've seen before include "Eg, bananas" , "Eg, apple or pear" , "Apples, pears etc" , "Kiwi, mandarin etc" , "Eg, banana or apple" .)