Small trading centre teens regularly tidy up (7)
I believe the answer is:
smarten
'tidy up' is the definition.
'small trading centre teens regularly' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'trading centre' becomes 'mart' (I've seen this before).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'teens' are 'en'.
's'+'mart'+'en'='SMARTEN'
(Other definitions for smarten that I've seen before include "Spruce up" , "Brighten (up)" , "Become bright, tidy and more efficient" , "Improve appearance" .)