Beginning to serve, drag shot (4)
I believe the answer is:
slug
'shot' is the definition.
(I've seen this before)
'beginning to serve drag' is the wordplay.
'beginning to' indicates taking the first letters.
'drag' becomes 'lug' (I've seen this in other clues).
The initial letter of 'serve' is 's'.
's'+'lug'='SLUG'
(Other definitions for slug that I've seen before include "Bullet - punch" , "Live" , "Heavy blow for snail lacking shell" , "gulp!" , "Terrestrial mollusc" .)