Agassi's first return of service is some way off (4)
I believe the answer is:
afar
'is some way off' is the definition.
(I've seen this before)
'agassi's first return of service' is the wordplay.
'agassi's first' becomes 'a' (1st letter of 'agassi').
'return of' says the letters should be written in reverse.
'service' becomes 'RAF' (Royal Air Force).
'raf' in reverse letter order is 'far'.
'a'+'far'='AFAR'
(Other definitions for afar that I've seen before include "long way back" , "Not in this neighbourhood" , "Way-out" , "A long distance away" , "from baseline to baseline?" .)