Block leftists finally in Tory gain (7)
I believe the answer is:
congest
'block' is the definition.
(I know that congest is a more specific form of block)
'leftists finally in tory gain' is the wordplay.
'finally' says to take the final letters.
'in' indicates putting letters inside.
'tory' becomes 'con' (abbreviation for Conservative).
'gain' becomes 'get' (I've seen this in another clue).
The last letter of 'leftists' is 's'.
'con'+'get'='conget'
's' inserted within 'conget' is 'CONGEST'.
(Other definitions for congest that I've seen before include "Become or cause to become obstructed" , "Block up" , "Become overcrowded" , "fill with traffic?" .)