Win fifty every second? That's dubious! (4)
I believe the answer is:
iffy
'dubious' is the definition.
(I've seen this in another clue)
'win fifty every second?' is the wordplay.
'every second?' indicates alternate letters (I've seen 'every second of' mean this).
'win'+'fifty'='winfifty'
The alternate letters of 'winfifty' are 'IFFY'.
'that's' acts as a link.
(Other definitions for iffy that I've seen before include "Dodgy" , "not kosher" , "Informally dubious or uncertain" , "Doubtful, not definite" , "Colloquially uncertain or doubtful" .)