Gamble about son being first (4)
I believe the answer is:
best
'being first' is the definition.
(similar in meaning)
'gamble about son' is the wordplay.
'gamble' becomes 'bet' (betting is a kind of gambling).
'about' means one lot of letters goes inside another.
'son' becomes 's' (genealogical abbreviation for son).
'bet' placed around 's' is 'BEST'.
(Other definitions for best that I've seen before include "Overcome" , "unbeatable" , "Beat" , "Superlative" , "Most excellent; outwit" .)