Open up one with sharp blow (6)
I believe the answer is:
unwrap
'open up' is the definition.
(thesaurus)
'one with sharp blow' is the wordplay.
'one' becomes 'un' (northern English dialect form of 'one').
'with' becomes 'w' (abbreviation).
'sharp blow' becomes 'rap' (rap can mean a sharp hit or blow).
'un'+'w'+'rap'='UNWRAP'
(Other definitions for unwrap that I've seen before include "Remove the covering from" , "Open (a gift)" , "Remove packaging from" , "Open (a present)" , "Undo" .)