Former helper is an expert (3,4)
I believe the answer is:
old hand
'an expert' is the definition.
(I know that old hand is a type of expert)
'former helper' is the wordplay.
'former' becomes 'old' (both can mean previous).
'helper' becomes 'hand' (I've seen this in another clue).
'old'+'hand'='OLD HAND'
'is' acts as a link.
(Other definitions for old hand that I've seen before include "Someone very experienced" , "Experienced performer" , "Veteran" , "mediaeval script" , "One's experienced" .)