Fancy a blonde, mate? (3,4)
I believe the answer is:
old bean
'mate?' is the definition.
(both are casual ways of addressing someone)
'fancy a blonde' is the wordplay.
'fancy' indicates an anagram.
'a'+'blonde'='ablonde'
'ablonde' with letters rearranged gives 'OLD BEAN'.
(Other definitions for old bean that I've seen before include "Dated familiar term of address to a man" , "Familiar way to address" , "Good friend" , "Who Wooster might have addressed" , "My dear chap!" .)