Lenin rued revolutionary stress (9)
I believe the answer is:
underline
'stress' is the definition.
(underlining is a kind of stressing)
'lenin rued revolutionary' is the wordplay.
'revolutionary' is an anagram indicator.
'lenin'+'rued'='leninrued'
'leninrued' anagrammed gives 'UNDERLINE'.
(Other definitions for underline that I've seen before include "To show emphasis in writing" , "Form of emphasis in writing" , "Give extra weight to, stress" , "Reinforce" , "Emphasise with horizontal stroke of pen" .)