Slice boundary following wide (5)
I believe the answer is:
wedge
'slice' is the definition.
(I've seen this before)
'boundary following wide' is the wordplay.
'boundary' becomes 'edge' ('edge' can be a synonym of 'boundary'**).
'following' means one lot of letters go next to another.
'wide' becomes 'w' (abbreviation e.g. in cricket).
'edge' put after 'w' is 'WEDGE'.
(Other definitions for wedge that I've seen before include "Tapered insert" , "V-sign" , "Lofted club" , "Triangular, thick piece" , "Type of shoe" .)