Falls back when tree rats active (8)
I believe the answer is:
retreats
'falls back' is the definition.
(I've seen this in another clue)
'tree rats active' is the wordplay.
'active' indicates anagramming the letters (the letters are active/moving).
'tree'+'rats'='treerats'
'treerats' is an anagram of 'RETREATS'.
'when' is the link.
(Other definitions for retreats that I've seen before include "Refuges" , "Recedes" , "Backs away" , "tactical withdrawals" , "Of army, falls back before enemy attack" .)