One who regrets hiding key for person delivering (7)
I believe the answer is:
rescuer
'person delivering' is the definition.
'one who regrets hiding key' is the wordplay.
'one who regrets' becomes 'ruer'.
'hiding' indicates putting letters inside.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'ruer' placed around 'esc' is 'RESCUER'.
'for' acts as a link.
(Other definitions for rescuer that I've seen before include "freer" , "One who saves another from peril" , "Saviour" , "One who delivers" , "One who saves someone" .)