Not really a shutter, nevertheless nearer the mark (6)
I believe the answer is:
closer
'not really a shutter nevertheless' is the definition.
I can't judge whether this definition defines the answer.
'nearer the mark' is the wordplay.
'nearer' becomes 'close' (similar in meaning).
'the mark' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'close'+'r'='CLOSER'
Can you help me to learn more?
(Other definitions for closer that I've seen before include "more reticent" , "Comparatively near" , "denser" , "One winds up" , "Cresol (anag.)" .)