A type of service penetrating the tennis player? (7)
I believe the answer is:
athlete
'tennis player?' is the definition.
(tennis player is a kind of athlete)
'a type of service penetrating the' is the wordplay.
'type of service' becomes 'let'.
'penetrating' means one lot of letters goes inside another.
'let' going within 'the' is 'thlete'.
'a'+'thlete'='ATHLETE'
(Other definitions for athlete that I've seen before include "Strangely, the tale of one who is fit" , "gymnast, maybe" , "The tale (anag)" , "Person fit for physical sports" , "One may run" .)