Family member spent so recklessly (7)
I believe the answer is:
stepson
'family member' is the definition.
(son of one's spouse)
'spent so recklessly' is the wordplay.
'recklessly' is an anagram indicator.
'spent'+'so'='spentso'
'spentso' anagrammed gives 'STEPSON'.
(Other definitions for stepson that I've seen before include "Tramples (5,2) -- relative" , "Male offspring of partner's former marriage" , "second family member" , "No pests (anag)" , "Subdues" .)