Screw cap alternative (4)
I believe the answer is:
cork
'screw cap alternative' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cork that I've seen before include "Stopper; Irish city" , "Bottle-stopper for a southern city" , "One stops" , "Bung for a bottle" , "Float" .)