Under arrest, run inside exit and cover "Fingers" with this? (4,7)
I believe the answer is:
nail varnish
'exit and cover fingers with this?' is the definition.
The definition suggests a singular noun which matches the answer.
'under arrest run inside' is the wordplay.
'under' says to put letters next to each other (in a down clue, some letters go below others).
'arrest' becomes 'nail' (synonyms).
'run inside' becomes 'varnish' (I can't justify this - if you can you should give a lot more credence to this answer).
'nail'+'varnish' is 'NAIL-VARNISH'.
Can you help me to learn more?
(Other definitions for nail varnish that I've seen before include "Lacquer applied to the body's extremities" , "Lacquer at one's fingertips" , "Manicurist's cosmetic lacquer" , "Make-up for fingers and toes" , "digital enhancement" .)