Theft occasionally leading to scrap on square perhaps (8)
I believe the answer is:
tetragon
'square perhaps' is the definition.
(I know that foursquare is a type of rectangle)
'theft occasionally leading to scrap on' is the wordplay.
'occasionally' indicates alternate letters.
'leading to' is a charade indicator (letters next to each other) (some letters go before or lead others).
'scrap' becomes 'rag' (I've seen this in another clue).
The alternating letters of 'theft' are 'tet'.
'tet'+'rag'+'on'='TETRAGON'
(Other definitions for tetragon that I've seen before include "Plane figure with four angles and sides" , "Shape" , "Four-sided plane figure" .)