Environmental indicator, say, for fruit (9)
I believe the answer is:
greengage
'for fruit' is the definition.
(type of plum)
'environmental indicator' is the wordplay.
'environmental' becomes 'green'.
'indicator' becomes 'gage' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'green'+'gage'='GREENGAGE'
'say' is the link.
Can you help me to learn more?
(Other definitions for greengage that I've seen before include "Plum-like fruit" , "Garden fruit" , "it's fruity" , "Such a summer novel" , "Fruit like a small plum" .)