Behind Tory leader sitting on fence (8)
I believe the answer is:
trailing
'behind' is the definition.
('trailing' can be a synonym of 'behind')
'tory leader sitting on fence' is the wordplay.
'tory leader' becomes 't' (I've seen this in other clues).
'sitting on' says to put letters next to each other.
'fence' becomes 'railing' (I've seen this before).
't'+'railing'='TRAILING'
(Other definitions for trailing that I've seen before include "at the back" , "Lagging behind in a race" , "Pursuing" , "Lagging behind or tracking" , "Following by using marks or scents left behind" .)