Core of students warning: stop environmental change (13)
I believe the answer is:
deforestation
'environmental change' is the definition.
'core of students warning stop' is the wordplay.
'core of' says to take the centre.
'warning' becomes 'fore' (I've seen this in other clues).
'stop' becomes 'station' (I've seen this before**).
The central letters of 'students' are 'de'.
'de'+'fore'+'station'='DEFORESTATION'
(Other definitions for deforestation that I've seen before include "Clearance of trees" , "Large-scale tree felling" .)