To be blunt, muscle was caught in zip (6)
I believe the answer is:
flatly
'to' is the definition.
Both the definition and answer are adverbs.
Perhaps there's a link between them I don't understand?
'blunt muscle was caught in zip' is the wordplay.
I cannot quite see how this works, but
'muscle' could be 'lat' (short for latissimus muscle) and 'lat' is found within the answer.
The remaining letters 'fly' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'be' acts as a link.
Can you help me to learn more?
(Other definitions for flatly that I've seen before include "in uninspiring fashion" , "Absolutely" , "(Deny) without qualification" , "Uncompromisingly (refuse)" , "point-blank" .)