Get better of old fool, nabbing last of Cointreau (6)
I believe the answer is:
outwit
'get better of' is the definition.
'old fool nabbing last of cointreau' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fool' becomes 'twit' (I've seen this before).
'nabbing' indicates putting letters inside.
'last of' suggests the final letters.
The last letter of 'cointreau' is 'u'.
'o'+'twit'='otwit'
'otwit' going around 'u' is 'OUTWIT'.
(Other definitions for outwit that I've seen before include "To fox" , "Mentally get the better of" , "Hoodwink" , "Show more ingenuity than" , "Get the better of by being smarter" .)