Fortune made by opportunist? Not quite (6)
I believe the answer is:
chance
'fortune' is the definition.
('chance' can be a synonym of 'fortune')
'opportunist? not quite' is the wordplay.
'opportunist?' becomes 'chancer' (I've seen this before).
'not quite' means to remove the last letter.
'chancer' with its final letter removed is 'CHANCE'.
'made by' is the link.
(Other definitions for chance that I've seen before include "Coincidence" , "Destiny, kismet" , "Likelihood" , "Fortune, hazard" , "Luck or accident" .)