One helping girl to avoid extremes (4)
I believe the answer is:
aide
'one helping' is the definition.
(an assistant or helper)
'girl to avoid extremes' is the wordplay.
'girl' becomes 'maiden' (maiden is a kind of girl).
'to avoid extremes' says to take the centre (the extremes of the word are removed).
The central letters of 'maiden' are 'AIDE'.
(Other definitions for aide that I've seen before include "Idea (anag)" , "Confidential assistant or advisor" , "Officer in attendance" , "Somebody helping" , "a help" .)