Not for the first time, shut up! (6)
I believe the answer is:
reseal
'up' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
'not for the first time shut' is the wordplay.
'not for the first time' becomes 're' (I can't explain this - if you can you should give a lot more credence to this answer).
'shut' becomes 'seal' (sealing is a kind of shutting).
're'+'seal'='RESEAL'
Can you help me to learn more?
(Other definitions for reseal that I've seen before include "Close securely again" , "Close again tightly" .)