Backing boy: a new tennis star (5)
I believe the answer is:
nadal
'tennis star' is the definition.
(tennis player Rafael Nadal)
'backing boy a new' is the wordplay.
'backing' says the letters should be written backwards.
'boy' becomes 'lad' (informal term for boy).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'lad'+'a'+'n'='ladan'
'ladan' in reverse letter order is 'NADAL'.
(Other definitions for nadal that I've seen before include "Star with a 30 [RACKET]" , "Top tennis player" , "tennis champion" , "court figure" , "Rafael __, Spanish tennis player" .)