Synthetic fibre or paint (7)
I believe the answer is:
acrylic
I believe this is a double definition.
'synthetic fibre' is the first definition.
(I've seen this before)
'paint' is the second definition.
(acrylic is a kind of paint)
(Other definitions for acrylic that I've seen before include "Type of paint or textile fibre" , "Type of hard paint" , "Type of twenty Dn" , "Pungent acid" , "Textile fibre - icy Carl (anag)" .)