Vague family member with hard heart (7)
I believe the answer is:
unclear
'vague' is the definition.
(similar in meaning)
'family member with hard heart' is the wordplay.
'family member' becomes 'uncle' (brother of a parent).
'with' is a charade indicator (letters next to each other).
'heart' means to look at the middle letters.
The middle of 'hard' is 'ar'.
'uncle'+'ar'='UNCLEAR'
(Other definitions for unclear that I've seen before include "Not transparent or free from doubt" , "Not obvious, obscure" , "Not apparent" , "Not certain" , "An ulcer (anag)" .)