Standard number assigned to soldiers (4)
I believe the answer is:
norm
'standard' is the definition.
(I know that standard can be written as norm)
'number assigned to soldiers' is the wordplay.
'number' becomes 'no.' (abbreviation).
'assigned to' means one lot of letters go next to another.
'soldiers' becomes 'rm' (Royal Marines).
'no'+'rm'='NORM'
(Other definitions for norm that I've seen before include "Average result" , "Standard, criterion" , "Standard considered to be representative" , "What is conventional, regular" , "expected thing" .)