Saver, one who regrets concealing key (7)
I believe the answer is:
rescuer
'saver' is the definition.
(I know that saver can be written as rescuer)
'one who regrets concealing key' is the wordplay.
'one who regrets' becomes 'ruer' (I've seen this in another clue).
'concealing' is an insertion indicator.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'ruer' enclosing 'esc' is 'RESCUER'.
(Other definitions for rescuer that I've seen before include "Deliverer (from danger)" , "One who delivers" , "Person bringing one out of danger" , "freer" , "one retrieves" .)