Trump thanks popular military leader (7)
I believe the answer is:
captain
'military leader' is the definition.
'trump thanks popular' is the wordplay.
'trump' becomes 'cap' (both can mean to beat or override).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'popular' becomes 'in' ('in' can mean trendy).
'cap'+'ta'+'in'='CAPTAIN'
(Other definitions for captain that I've seen before include "Lead (a team)" , "Army rank" , "Commander of a ship" , "American police officer" , "Ship's commander" .)