Plant secretion used by engineers, southern, at home (5)
I believe the answer is:
resin
'plant secretion' is the definition.
(I've seen this in another clue)
'by engineers southern at home' is the wordplay.
'by' is a charade indicator (letters next to each other).
'engineers' becomes 're' (Royal Engineers).
'southern' becomes 's' (abbreviation).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
're' next to 's' is 'res'.
'res'+'in'='RESIN'
'used' acts as a link.
(Other definitions for resin that I've seen before include "Sticky tree exudation" , "Sticky stuff as from pine tree" , "Synthetic substance" , "Organic polymer used in varnishes, adhesives etc." , "Sticky stuff from pine trees" .)