Gamble on self-centred nut (5)
I believe the answer is:
betel
'nut' is the definition.
(I've seen this in another clue)
'gamble on self-centred' is the wordplay.
'gamble' becomes 'bet' (betting is a kind of gambling).
'on' says to put letters next to each other.
'centred' means to look at the middle letters.
The central letters of 'self' are 'el'.
'bet'+'el'='BETEL'
(Other definitions for betel that I've seen before include "stimulatingly chewed in Asia" , "Asian palm and nut" , "climbing plant" , "Narcotic leaf" , "Palm with nuts for chewing" .)