Legend, very generous sort, Mrs Thatcher — as if! (6,9)
I believe the answer is:
father christmas
'legend very generous' is the definition.
(legendary generous figure)
'sort mrs thatcher as if' is the wordplay.
'sort' indicates an anagram.
'mrs'+'thatcher'+'as'+'if'='mrsthatcherasif'
'mrsthatcherasif' with letters rearranged gives 'FATHER CHRISTMAS'.
(Other definitions for father christmas that I've seen before include "he'll be sacked" , "man with gifts" , "late December he's busy" , "present supremo" , "Santa!" .)