Starts to bash excellent tennis stroke in anticipation (10)
I believe the answer is:
beforehand
'in anticipation' is the definition.
(thesaurus)
'starts to bash excellent tennis stroke' is the wordplay.
'starts to' says to take the initial letters.
'tennis stroke' becomes 'forehand' (forehand is a kind of tennis stroke**).
The initial letters of 'bash excellent' is 'be'.
'be'+'forehand'='BEFOREHAND'
(Other definitions for beforehand that I've seen before include "Earlier" , "In advance" , "Previously" .)