Relation had urged grant must be modified (13)
I believe the answer is:
granddaughter
'relation' is the definition.
(granddaughter is a kind of relation)
'had urged grant must be modified' is the wordplay.
'must be modified' indicates an anagram.
'had'+'urged'+'grant'='hadurgedgrant'
'hadurgedgrant' is an anagram of 'GRANDDAUGHTER'.
(Other definitions for granddaughter that I've seen before include "gives shop-owner's description of Nell Trent" , "Relation" .)