Head of regiment drills joint force (5)
I believe the answer is:
drive
'force' is the definition.
(driving is a kind of forcing)
'head of regiment drills joint' is the wordplay.
'head of' indicates taking the first letters.
'drills' is an insertion indicator (to drill can mean to make a hole in something).
'joint' becomes 'dive' (I've seen this before).
The first letter of 'regiment' is 'r'.
'r' going inside 'dive' is 'DRIVE'.
(Other definitions for drive that I've seen before include "Sales campaign - energy - push" , "Ambition - steer" , "Steer; trip" , "Car journey; urge" , "Strong motivation, ambition" .)