Environmental travel is a pest (8)
I believe the answer is:
greenfly
'pest' is the definition.
(I've seen this before)
'environmental travel' is the wordplay.
'environmental' becomes 'green'.
'travel' becomes 'fly' (flying is a kind of travelling).
'green'+'fly'='GREENFLY'
'is a' is the link.
(Other definitions for greenfly that I've seen before include "Common aphid" , "Pests" , "Common aphids" , "Garden pest" .)