About to stop low-down tree-hugger? (5)
I believe the answer is:
green
'tree-hugger?' is the definition.
(both can mean an environmental activist)
'about to stop low-down' is the wordplay.
'about' becomes 're' (common abbreviation for 'with reference to').
'to stop' is an insertion indicator.
'low down' becomes 'gen' (I've seen this in other clues).
're' inserted inside 'gen' is 'GREEN'.
(Other definitions for green that I've seen before include "Lacking experience (on the golf course?)" , "Politician; fresh" , "Unripe; envious" , "Colour of emeralds" , "part of the golf course" .)