Harmless cousin sound at heart possibly (9)
I believe the answer is:
innocuous
'harmless' is the definition.
('innocuous' can be a synonym of 'harmless')
'cousin sound at heart possibly' is the wordplay.
'at heart' means to look at the middle letters.
'possibly' indicates anagramming the letters.
The centre of 'sound' is 'oun'.
'cousin'+'oun'='cousinoun'
'cousinoun' is an anagram of 'INNOCUOUS'.
(Other definitions for innocuous that I've seen before include "Not harmful or offensive" , "Harmless" .)