Doctor isn’t admitting mislaying coat? (5,5)
I believe the answer is:
gloss paint
'coat?' is the definition.
(I have seen 'Shiny coat' mean 'gloss paint' so perhaps 'coat' could also mean 'gloss paint')
'doctor isn't admitting mislaying' is the wordplay.
'doctor' becomes 'GP' (general practitioner).
'isn't' becomes 'aint' (non-standard version of 'isn't').
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'mislaying' becomes 'loss' (loss can mean mislaying something).
'gp'+'aint'='gpaint'
'gpaint' placed around 'loss' is 'GLOSS PAINT'.
(Other definitions for gloss paint that I've seen before include "Shiny covering" , "Shiny coat" .)