Drill first of recruits in section (6)
I believe the answer is:
pierce
'drill' is the definition.
(both can mean to go through something)
'first of recruits in section' is the wordplay.
'first of recruits' becomes 'r' (1st letter of 'recruits').
'in' means one lot of letters goes inside another.
'section' becomes 'piece' (I've seen this before).
'r' placed within 'piece' is 'PIERCE'.
(Other definitions for pierce that I've seen before include "Punch" , "Penetrate sharply" , "Enter" , "Prick" , "Make hole in using pointed instrument" .)