Just about setter on top (4)
I believe the answer is:
mere
'just' is the definition.
(I've seen this in another clue)
'about setter on top' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'setter' becomes 'me' (the setter of the clue).
'on top' means one lot of letters go next to another (in a down clue, some letters go above others).
're' put after 'me' is 'MERE'.
(Other definitions for mere that I've seen before include "Lake - French mother" , "Nothing more than a small pond" , "Previously pure" , "Pure; lake" , "DDwater" .)