Regret being stopped by key save (6)
I believe the answer is:
rescue
'save' is the definition.
(rescuing is a kind of saving)
'regret being stopped by key' is the wordplay.
'regret' becomes 'rue' (to rue something is to regret it).
'being stopped by' indicates putting letters inside (stopping can mean blocking a gap).
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'rue' placed around 'esc' is 'RESCUE'.
(Other definitions for rescue that I've seen before include "Help out of difficulty, save" , "Free from danger or captivity" , "Save from danger or distress" , "Being presumptuous" , "Free - recovery" .)