Tramp's son turned out to be an athlete (9)
I believe the answer is:
sportsman
'athlete' is the definition.
(I know that athlete is a type of athlete)
'tramp's son turned out' is the wordplay.
'turned out' indicates an anagram (turn out can mean to organise).
'tramps'+'son'='trampsson'
'trampsson' anagrammed gives 'SPORTSMAN'.
'to be an' acts as a link.
(Other definitions for sportsman that I've seen before include "Male athlete" , "Chap aiming for game" , "Eg, athlete" , "He engages in physical games" , "Rower perhaps" .)