Leading Conservative may be standing (7)
I believe the answer is:
upright
'standing' is the definition.
(I've seen this before)
'leading conservative' is the wordplay.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'conservative' becomes 'right' ('right' can be similar in meaning to 'conservative').
'up'+'right'='UPRIGHT'
'may be' is the link.
(Other definitions for upright that I've seen before include "Vertical, erect" , "Pole; principled" , "Vertical post in rugby goal - strictly honourable" , "Vertical like rugby goal post" , "Un-grand piano" .)