A coat for one to gasp about! (5)
I believe the answer is:
paint
'a coat' is the definition.
(I know this)
'one to gasp about' is the wordplay.
'one' becomes 'I' (Roman numeral).
'to gasp' becomes 'pant' (synonyms).
'about' is an insertion indicator.
'i' placed into 'pant' is 'PAINT'.
'for' is the link.
(Other definitions for paint that I've seen before include "Coloured coating" , "Item used by 23 Across" , "Use a brush on canvas" , "Apply a coat of colour" , "Decorative coating" .)