Your leader supports action to make capital that's dubious (6)
I believe the answer is:
shifty
'dubious' is the definition.
(I've seen this before)
'your leader supports action to make capital' is the wordplay.
'leader' suggests taking the first letters.
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'action to make capital' becomes 'shift' (the shift button on a keyboard turns capitals on).
The first letter of 'your' is 'y'.
'y' put after 'shift' is 'SHIFTY'.
'that's' acts as a link.
(Other definitions for shifty that I've seen before include "Furtive" , "Sneaky and sly" , "Constantly changing; deceitful, evasive" , "Appearing evasive" , "Suspicious-looking" .)