Loosen, as a cap (7)
I believe the answer is:
untwist
'loosen as a cap' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for untwist that I've seen before include "Do this removing cap" , "Dishevel" , "free of entanglements" , "Sort out - release (cap on a plastic bottle?)" , "Perform opening" .)