Oomph; fastener (3)
I believe the answer is:
zip
I believe this clue is a double definition.
'oomph' is the first definition.
(I've seen this in another clue)
'fastener' is the second definition.
(I know that zip is a type of fastener)
(Other definitions for zip that I've seen before include "drive" , "Locking device with two toothed edges and a sliding tab" , "Energy or clothes fastener" , "Fastening device" , "required in open-and-shut case?" .)