Is no longer hot in thin coat (4)
I believe the answer is:
wash
'thin coat' is the definition.
(thin coat of paint)
'is no longer hot' is the wordplay.
'is no longer' becomes 'was' (eg if someone 'was' sad, they're no longer sad).
'hot' becomes 'h' (abbreviation eg on taps).
'was'+'h'='WASH'
'in' acts as a link.
(Other definitions for wash that I've seen before include "Perform ablutions" , "Of animals, cleanse by licking" , "Rinse" , "Bathe" , "Clean with water" .)