Silent relation (3)
I believe the answer is:
mum
I believe this clue is a double definition.
'silent' is the first definition.
(synonyms)
'relation' is the second definition.
(mum is a kind of relation)
(Other definitions for mum that I've seen before include "(Keep) silent; mother" , "Silent; parent" , "Keep it secret from mother?" , "Palindromic mater" , "Parent - shtoom" .)