Mum has cold, needing raincoat (3)
I believe the answer is:
mac
'raincoat' is the definition.
(macintosh is a type of raincoat)
'mum has cold' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'has' is a charade indicator (letters next to each other).
'cold' becomes 'c' (eg on taps).
'ma'+'c'='MAC'
'needing' acts as a link.
(Other definitions for mac that I've seen before include "It may help you keep dry" , "A Gaelic son, often in name" , "A rainproof" , "Coat - computer" , "one used in shower?" .)