Regular soldiers assume it (7)
I believe the answer is:
uniform
'regular' is the definition.
(similar in meaning)
'soldiers assume it' is the wordplay.
I cannot really understand how this works, but
'soldiers' could be 'rm' (Royal Marines) and 'rm' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for uniform that I've seen before include "Unchanging, showing no variation" , "what soldier wears?" , "Homogeneous" , "Military tunic" , "Unvarying - work clothing" .)