Most successful, getting shilling in wager (4)
I believe the answer is:
best
'most successful' is the definition.
'shilling in wager' is the wordplay.
'shilling' becomes 's'.
'in' indicates putting letters inside.
'wager' becomes 'bet' ('bet' can be a synonym of 'wager'**).
's' placed inside 'bet' is 'BEST'.
'getting' acts as a link.
(Other definitions for best that I've seen before include "The most talented" , "Most advantageous" , "unbeatable" , "Superlative" , "former football star" .)