A good service was one of her strengths (4)
I believe the answer is:
graf
'one of her strengths' is the definition.
I can't judge whether this defines the answer.
'a good service' is the wordplay.
'a good' becomes 'g' (abbreviation).
'service' becomes 'RAF' (Royal Air Force).
'g'+'raf'='GRAF'
'was' acts as a link.
(Other definitions for graf that I've seen before include "Steffi --, tennis player" , "German tennis player, b. 1969" , "Steffi --, tennis champion" , "German count - German tennis player" , "Seven-times women's Wimbledon champion, b. 1969" .)