Polish cop to go AWOL, crossing river (4,7)
I believe the answer is:
nail varnish
'polish' is the definition.
'cop to go awol crossing river' is the wordplay.
'cop' becomes 'nail' ('nail' can be a synonym of 'cop'**).
'to go awol' becomes 'vanish'.
'crossing' is an insertion indicator.
'river' becomes 'r'.
'vanish' placed around 'r' is 'varnish'.
'nail'+'varnish'='NAIL-VARNISH'
(Other definitions for nail varnish that I've seen before include "digital enhancement" , "Lacquer at one's fingertips" , "Manicurist's cosmetic lacquer" , "Make-up for fingers and toes" , "Lacquer applied to the body's extremities" .)