Third of Tories wearing blue go under (5)
I believe the answer is:
drown
'go under' is the definition.
(I've seen this before)
'third of tories wearing blue' is the wordplay.
'third of tories' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'wearing' is an insertion indicator.
'blue' becomes 'down' (I've seen this before).
'r' inserted into 'down' is 'DROWN'.
(Other definitions for drown that I've seen before include "Perish in water" , "Die by immersion in liquid" , "Submerge, maybe fatally" , "Die in sea" , "Die from submersion in liquid" .)