More stuffy - its the shutter! (6)
I believe the answer is:
closer
'more stuffy' is the definition.
('close' can be a synonym of 'stuffy')
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for closer that I've seen before include "Stuffier" , "within a shorter distance" , "Not so far off" , "One who is skilled at bringing a business transaction to a satisfactory conclusion" , "Less distant" .)