Straighten out (6)
(Another definition for uncurl that I've seen is " Straighten from rolled up position".)
(Other definitions for untwine that I've seen before include "Become unwound - wine nut (anag)" , "Unwind; untwist" , "Take the string off" , "Disentangle" , "Ravel to be dealt with" .)
'straighten out' is the definition.
(untwisting is a kind of straightening out)
This is the entire clue.
(Other definitions for untwist that I've seen before include "free of entanglements" , "Dishevel" , "Sort out - release (cap on a plastic bottle?)" , "Perform opening" , "Straighten out" .)
(Other definitions for untangle that I've seen before include "Make free from confusion" , "'Unpick, unravel (8)'" , "Straighten out, free from mesh" , "Extricate" , "Sort out" .)