I must wear grey, being doomed? (5)
I believe the answer is:
goner
'being doomed?' is the definition.
(a goner is a doomed person)
'i must wear grey' is the wordplay.
'i' becomes 'one' (Roman numeral for one).
'must wear' means one lot of letters goes inside another.
'grey' becomes 'gr' (I've seen this in other clues).
'one' placed within 'gr' is 'GONER'.
(Other definitions for goner that I've seen before include "who's asleep" , "Something beyond any chance of recovery" , "He's unlikely to recover" , "Nasty princess" , "Article ruined" .)