Mum with cold needs waterproof (3)
I believe the answer is:
mac
'waterproof' is the definition.
(mac is a kind of waterproof)
'mum with cold' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'with' is a charade indicator (letters next to each other).
'cold' becomes 'c' (eg on taps).
'ma'+'c'='MAC'
'needs' acts as a link.
(Other definitions for mac that I've seen before include "laptop?" , "A rainproof" , "Waterproof raincoat, in short" , "Raincoat (colloq.)" , "Overcoat" .)