Square, popular in Tyneside area (4)
I believe the answer is:
nine
'square' is the definition.
(I've seen this before)
'popular in tyneside area' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'in' indicates putting letters inside.
'tyneside area' becomes 'ne' (North East of England).
'in' going within 'ne' is 'NINE'.
(Other definitions for nine that I've seen before include "Square next up after four" , "3x3" , "Days Lady Jane 5 ac reigned" , "Square number" , "One over the eight" .)