Brilliant odds against Close wearing a hat (8)
I believe the answer is:
splendid
'brilliant' is the definition.
(I've seen this before)
'odds against close wearing a hat' is the wordplay.
'odds' becomes 'sp' (betting abbreviation for starting price).
'against' means one lot of letters go next to another.
'close' becomes 'end' (I've seen this before).
'wearing' means one lot of letters goes inside another.
'a hat' becomes 'lid' ('lid' can be a synonym of 'hat').
'end' inserted within 'lid' is 'lendid'.
'sp'+'lendid'='SPLENDID'
(Other definitions for splendid that I've seen before include "Excellent, magnificent" , "Characterised by grandeur" , "Excellent, first-class" , "Great" , "Very fine, glorious" .)