Essentially cheap, lousy type of resin (5)
I believe the answer is:
epoxy
'type of resin' is the definition.
(I know that epoxy is a type of synthetic resin)
'essentially cheap lousy' is the wordplay.
'essentially' means to look at the middle letters.
'lousy' becomes 'poxy'.
The middle letter of 'cheap' is 'e'.
'e'+'poxy'='EPOXY'
(Other definitions for epoxy that I've seen before include "Type of resin" , "Adhesive; plastic" , "Synthetic adhesive, paint etc." , "Type of adhesive/paint" , "Hard-setting resin" .)