A quiet mother is plain (8)
I believe the answer is:
apparent
'is plain' is the definition.
(synonyms)
'a quiet mother' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'mother' becomes 'parent' (I've seen this in other clues).
'a'+'p'+'parent'='APPARENT'
(Other definitions for apparent that I've seen before include "Pane part may be quite obvious" , "Seeming real or true" , "Clearly visible" , "See 1" , "as such an heir" .)