Segregate fat soldiers coming in (4,3)
I believe the answer is:
sort out
'segregate' is the definition.
(thesaurus)
'fat soldiers coming in' is the wordplay.
'fat' becomes 'stout' ('stout' can be similar in meaning to 'fat'**).
'soldiers' becomes 'or' (Other Rank).
'coming in' is an insertion indicator.
'stout' going around 'or' is 'SORT OUT'.
(Other definitions for sort out that I've seen before include "Segregate and put in order" , "Resolve - to put in order" , "Resolve (a problem)" , "clear up" , "Group" .)