One delivering frightful curse runs after earl (7)
I believe the answer is:
rescuer
'one delivering' is the definition.
(deliver can mean to rescue or save)
'frightful curse runs after earl' is the wordplay.
'frightful' indicates an anagram (a frightful spelling).
'runs' becomes 'r' (cricket abbreviation).
'after' means one lot of letters go next to another.
'earl' becomes 'e'.
'curse' with letters rearranged gives 'rescu'.
'r' put after 'e' is 'er'.
'rescu'+'er'='RESCUER'
(Other definitions for rescuer that I've seen before include "One who saves from a dangerous situation" , "One who delivers" , "Deliverer (from danger)" , "One who saves another from peril" , "One who saves someone" .)