Resin base (7)
I believe the answer is:
polymer
'resin base' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for polymer that I've seen before include "Repeated-molecule compound" , "Mo, reply to the large molecule" , "Substance such as nylon" , "Eg, protein, rubber etc." , "Substance with large molecules of smaller units" .)