As a singer, kept one's mouth shut (6)
I believe the answer is:
hummed
'shut' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives sometimes mean the same thing.
'as a singer kept one's mouth' is the wordplay.
I cannot quite see how this works, but
'one' could be 'me' (a setter might use 'one' to humourously mean 'me') and 'me' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for hummed that I've seen before include "Sang with closed lips" , "Sang without words for the bees" , "stank" .)