Optimistic lad embraces union worker (7)
I believe the answer is:
buoyant
'optimistic' is the definition.
(thesaurus)
'lad embraces union worker' is the wordplay.
'lad' becomes 'boy' (I've seen this before).
'embraces' means one lot of letters goes inside another.
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'worker' becomes 'ant' (worker is a type of ant).
'boy' placed around 'u' is 'buoy'.
'buoy'+'ant'='BUOYANT'
(Other definitions for buoyant that I've seen before include "Floatable" , "Lively nineteen twenties dance" , "Cheerful, optimistic" , "refusing to go to bed?" , "Able to float" .)