Mother has what could be Chelsea dress (4)
I believe the answer is:
maxi
'dress' is the definition.
(I've seen this before)
'mother has what could be chelsea' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'has' is a charade indicator (letters next to each other).
'what could be chelsea' becomes 'XI' (XI can mean a football eleven).
'ma'+'xi'='MAXI'
(Other definitions for maxi that I've seen before include "Full-length skirt" , "Long skirt of coat" , "Relatively long skirt" , "racing yacht" , "full-length garment" .)