Green is erratic, partly reflecting amber? (5)
I believe the answer is:
resin
'amber?' is the definition.
(amber is a kind of resin)
'green is erratic partly reflecting' is the wordplay.
'partly' indicates a hidden word (part of the clue is used).
'reflecting' says the letters should be written in reverse.
'niser' is hidden in the letters of 'green is erratic'.
'niser' reversed gives 'RESIN'.
(Other definitions for resin that I've seen before include "Risen (anag.)" , "eg Rosin" , "Exudation from fir trees" , "Contributor to plastics industry" , "Material exuded from tree" .)