Voucher for Christmas often upset prickly old Nanna at first (6)
I believe the answer is:
coupon
'voucher' is the definition.
(I know that voucher can be written as coupon)
'christmas often upset prickly old nanna at first' is the wordplay.
'at first' says to take the initial letters.
The initial letters of 'christmas often upset prickly old nanna' is 'COUPON'.
'for' acts as a link.
(Other definitions for coupon that I've seen before include "Token" , "Voucher or ticket" , "means to buy goods" , "Entry form" , "betting slip" .)