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' says the answer is hidden in the clue (part of the clue is used).
'reflecting' shows that the letters should be reversed in order.
'niser' can be found hidden inside '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" .)