Grass going back inside garden wall (4)
I believe the answer is:
lawn
'grass' is the definition.
(I've seen this before)
'going back inside garden wall' is the wordplay.
'going back' says the letters should be written backwards.
'inside' says the answer is hidden in the clue.
'nwal' is hidden in the letters of 'garden wall'.
'nwal' backwards is 'LAWN'.
(Other definitions for lawn that I've seen before include "Fine cotton or grass garden" , "Grass-covered ground" , "Green area" , "Area of short grass" , "Patch of grass in the garden" .)