Helper paid extra in part (4)
I believe the answer is:
aide
'helper' is the definition.
(aide is a kind of helper)
'paid extra in part' is the wordplay.
'in part' indicates the answer is hidden within the clue.
'AIDE' can be found hidden inside 'paid extra'.
(Other definitions for aide that I've seen before include "Idea (anag)" , "Assistant to a more senior person" , "auxiliary" , "Leader's assistant" , "Officer in attendance" .)