Wide rivet passed through muzzle (6)
I believe the answer is:
gaping
'wide rivet passed through muzzle' is the definition.
I can't judge whether this definition defines the answer.
'passed through muzzle' is the wordplay.
'passed' becomes 'pin' (I can't justify this - if you can you should give a lot more credence to this answer).
'through' is an insertion indicator (inserted letters go through the word).
'muzzle' becomes 'gag' (synonyms).
'pin' put into 'gag' is 'GAPING'.
Can you help me to learn more?
(Other definitions for gaping that I've seen before include "Yawning" , "With the mouth wide open, in wonder perhaps" , "Staring open-mouthed" , "Opening mouth wide or staring in wonder" .)