Son winning place, in theory (11)
I believe the answer is:
supposition
'theory' is the definition.
(supposition is a kind of theory)
'son winning place' is the wordplay.
'son' becomes 's'.
'winning' becomes 'up' (I've seen this in other clues).
'place' becomes 'position' ('position' can be a synonym of 'place'**).
's'+'up'+'position'='SUPPOSITION'
'in' acts as a link.
(Other definitions for supposition that I've seen before include "Surmise" , "conjecture" , "Assumption, hypothesis" , "theory" , "It's taken for granted" .)