Be generous when cheated out of a fast finish (6)
I believe the answer is:
donate
'be generous' is the definition.
(I've seen this in another clue)
'cheated out of a fast finish' is the wordplay.
'cheated' becomes 'done' ().
'out of' means one lot of letters goes inside another (some letters on the outside of others).
'finish' suggests the final letters.
The final letter of 'fast' is 't'.
'a'+'t'='at'
'done' going around 'at' is 'DONATE'.
'when' is the link.
(Other definitions for donate that I've seen before include "Give (transplant organ)" , "Give to a charity or good cause" , "Atoned (anag)" , "Give freely, maybe blood" , "give in kindness" .)