Netting son installed in border going west (4)
I believe the answer is:
mesh
'netting' is the definition.
(I've seen this in another clue)
'son installed in border going west' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'installed in' means one lot of letters goes inside another.
'border' becomes 'hem' (I've seen this before).
'going west' shows that the letters should be reversed in order (letters heading to the west or left).
's' placed into 'hem' is 'hsem'.
'hsem' backwards is 'MESH'.
(Other definitions for mesh that I've seen before include "Web" , "Become entangled - become engaged" , "Net, snare" , "Piece of netting" , "Grid, netting" .)