Quiet mimic has form (5)
I believe the answer is:
shape
'form' is the definition.
(I know that form can be written as shape)
'quiet mimic' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'mimic' becomes 'ape' (ape can mean to copy or imitate).
'sh'+'ape'='SHAPE'
'has' is the link.
(Other definitions for shape that I've seen before include "Form, figure" , "Phase (anag.)" , "For good physical condition" , "Good physical form" , "'Figure, outline (5)'" .)