Cook buys a larger sweet (6,5)
I believe the answer is:
barley sugar
'sweet' is the definition.
(barley-sugar is a kind of sweet)
'cook buys a larger' is the wordplay.
'cook' indicates an anagram.
'buys'+'a'+'larger'='buysalarger'
'buysalarger' with letters rearranged gives 'BARLEY-SUGAR'.
(Other definitions for barley sugar that I've seen before include "Confection (originally made from a cereal)" , "Amber-coloured sweet" , "Twisted sweetmeat" , "Real as Rugby (anag) - sweet" .)