Block wife reaching border (5)
I believe the answer is:
wedge
'block' is the definition.
(wedge is a kind of block)
'wife reaching border' is the wordplay.
'wife' becomes 'w'.
'reaching' says to put letters next to each other.
'border' becomes 'edge' ('edge' can be a synonym of 'border').
'w'+'edge'='WEDGE'
(Other definitions for wedge that I've seen before include "Tapering object pushed into a gap - golf club" , "Chunk" , "piece of cake" , "Golf club, V-shape" , "Piece of wood thick at one end, thin at other" .)