Backed youth with an expert in court (5)
I believe the answer is:
nadal
'expert in court' is the definition.
(Rafael Nadal - player in a tennis court)
'backed youth with an' is the wordplay.
'backed' says the letters should be written backwards.
'youth' becomes 'lad' (I've seen this before).
'with' means one lot of letters go next to another.
'lad'+'an'='ladan'
'ladan' reversed gives 'NADAL'.
(Other definitions for nadal that I've seen before include "Star with a 30 [RACKET]" , "court figure" , "Rafael __, Spanish tennis player" , "tennis champion" , "Top tennis player" .)