Vegetation left that's fenced off by councils (5)
I believe the answer is:
flora
'vegetation' is the definition.
(I know that vegetation can be written as flora)
'left that's fenced off by councils' is the wordplay.
'left' becomes 'l' (common abbreviation).
'that's fenced off by' indicates putting letters inside.
'councils' becomes 'fora' (more than one forum).
'l' inserted into 'fora' is 'FLORA'.
(Other definitions for flora that I've seen before include "Form of life" , "Plants of a region or age" , "Plant life of a given place" , "list of plants" , "vegetation" .)