Outdo mostly loyal politician (5)
I believe the answer is:
trump
'outdo' is the definition.
('trump' can be a synonym of 'outdo')
'mostly loyal politician' is the wordplay.
'mostly' means to remove the last letter.
'loyal' becomes 'true' (I've seen this before).
'politician' becomes 'mp'.
'true' with its last letter removed is 'tru'.
'tru'+'mp'='TRUMP'
(Other definitions for trump that I've seen before include "reliable person" , "club perhaps" , "A playing card given added value in some games" , "High-ranking card" , "Card from chosen suit in various card games" .)