Block boundary after wide (5)
I believe the answer is:
wedge
'block' is the definition.
(wedge is a kind of block)
'boundary after wide' is the wordplay.
'boundary' becomes 'edge' (synonyms).
'after' says to put letters next to each other.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'edge' put after 'w' is 'WEDGE'.
(Other definitions for wedge that I've seen before include "Force (into narrow space); type of golf club" , "Tapering block" , "Tapered block" , "Triangular, thick piece" , "Piece of wood thick at one end, thin at other" .)