Small trading centre teens regularly tidy up (7)
I believe the answer is:
Here is my best explanation:
'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' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'teens' are 'en'.
(Other definitions for smarten that I've seen before include "Spruce up", "Brighten (up)", "brighten up", "Become bright, tidy and more efficient" and "Improve appearance".)