Soldiers in part of regiment (3)
I believe the answer is:
men
'soldiers' is the definition.
(men can mean a group of soldiers)
'part of regiment' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'MEN' is hidden in the letters of 'regiment'.
'in' is the link.
(Other definitions for men that I've seen before include "Male humans" , "Crew" , "Chess pieces" , "Males of Armenian extraction." , "Fellows, chaps" .)