A good area -- and popular once more (5)
I believe the answer is:
again
'once more' is the definition.
('again' can be a synonym of 'once more')
'a good area and popular' is the wordplay.
'a good' becomes 'ag' (I've seen this in other clues).
'area' becomes 'a' (maths abbreviation).
'and' says to put letters next to each other.
'popular' becomes 'in'.
'ag'+'a'+'in'='AGAIN'
(Other definitions for again that I've seen before include "repeat order" , "'Another time, once more (5)'" , "On the other hand" , "Repeatedly" , "Next time" .)