Army unit and police crossing river (5)
I believe the answer is:
corps
'army' is the definition.
(thesaurus)
'unit and police crossing river' is the wordplay.
'unit' becomes 'c' (C is an example).
'and' means one lot of letters go next to another.
'police' becomes 'ops' (I've seen this in other clues**).
'crossing' means one lot of letters goes inside another.
'river' becomes 'r'.
'ops' enclosing 'r' is 'orps'.
'c'+'orps'='CORPS'
(Other definitions for corps that I've seen before include "Army grouping" , "Large army group" , "Specialised mil. body" , "gang" , "Body of men" .)