Camera mechanism — extremely smooth, say (7)
I believe the answer is:
shutter
'camera mechanism' is the definition.
(I've seen this before)
'extremely smooth say' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'say' becomes 'utter' (to utter something is to say or speak it).
'smooth' with its centre taken out is 'sh'.
'sh'+'utter'='SHUTTER'
(Other definitions for shutter that I've seen before include "Moveable window screen" , "Device on a camera, it opens and closes" , "camera component" , "Camera mechanism" , "Removable cover" .)