Tramp's son developed into an athlete (9)
I believe the answer is:
sportsman
'an athlete' is the definition.
(sportsman is a kind of athlete)
'tramp's son developed' is the wordplay.
'developed' indicates anagramming the letters.
'tramps'+'son'='trampsson'
'trampsson' with letters rearranged gives 'SPORTSMAN'.
'into' is the link.
(Other definitions for sportsman that I've seen before include "Rower perhaps" , "Game fellow" , "player" , "Eg, athlete" , "Male athlete" .)