Joe wearing raincoat? Marvellous! (5)
I believe the answer is:
magic
'marvellous' is the definition.
(I've seen this before)
'joe wearing raincoat?' is the wordplay.
'joe' becomes 'gi' (GI Joe).
'wearing' is an insertion indicator.
'raincoat?' becomes 'mac' (mac is a kind of raincoat).
'gi' going into 'mac' is 'MAGIC'.
(Other definitions for magic that I've seen before include "Tricks" , "Conjuror's art" , "Unexpectedly produce" , "Art that supposedly invokes supernatural powers" , "Fungus causing trips?" .)