Doubt regarding Federer's first shot? (7)
I believe the answer is:
reserve
'doubt' is the definition.
(both can mean a feeling of hesitancy)
'regarding federer's first shot?' is the wordplay.
'regarding' becomes 'Re'.
'federer's first shot?' becomes 'serve' (shot in tennis - Roger Federer is a tennis player).
're'+'serve'='RESERVE'
(Other definitions for reserve that I've seen before include "some distance" , "put to one side" , "keep for oneself" , "Keep back - book" , "Husband - substitute" .)