Red setter chasing his tail in plot (6)
I believe the answer is:
scheme
'plot' is the definition.
(I know that scheme is a more specific form of plot)
'red setter chasing his tail' is the wordplay.
'red' becomes 'Che' (Che Guevara the Marxist revolutionary).
'setter' becomes 'me' (the setter of the clue).
'chasing' says to put letters next to each other (some letters chase after others).
'tail' indicates one should take the final letters.
The final letter of 'his' is 's'.
'che'+'me'='cheme'
'cheme' after 's' is 'SCHEME'.
'in' acts as a link.
(Other definitions for scheme that I've seen before include "Cunning plan" , "Plan (maybe best laid?)" , "proposal" , "Systematic of crafty plan of action" , "Work out a course of action" .)