Quiet answer -- Charles, perhaps, is trembling (7)
I believe the answer is:
shaking
'is trembling' is the definition.
(I've seen this before)
'quiet answer charles' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'answer' becomes 'a' (abbreviation eg in Q and A).
'charles' becomes 'king' (Charles I or Charles II).
'sh'+'a'+'king'='SHAKING'
'perhaps' is the link.
(Other definitions for shaking that I've seen before include "Atremble" , "Trembling, vibrating" .)