A man in raincoat - one having pasta (8)
I believe the answer is:
macaroni
'pasta' is the definition.
(I know that macaroni is a type of pasta)
'a man in raincoat one' is the wordplay.
'a man' becomes 'aron' ().
'in' is an insertion indicator.
'raincoat' becomes 'mac' (macintosh is a type of raincoat).
'one' becomes 'i' (Roman numeral).
'mac'+'i'='maci'
'aron' placed into 'maci' is 'MACARONI'.
'having' acts as a link.
(Other definitions for macaroni that I've seen before include "Slender tubes of pasta" , "Pasta variety, eighteenth century dandy" , "Hollow pasta" , "Tubed pasta" , "Alternative to 31 [TORTELLINI]" .)