Be first to support king, but not queen (5)
I believe the answer is:
exist
'be' is the definition.
('exist' can be a synonym of 'be')
'first to support king but not queen' is the wordplay.
'first to' becomes 'ist' (I can't justify this - if you can you should believe this answer much more).
'support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'king' becomes 'rex' (I've seen this before).
'but not' indicates named letters should be taken away.
'queen' becomes 'r' (r is abbreviation regina. e.g. in ER ).
'rex' with 'r' removed is 'ex'.
'ist' put after 'ex' is 'EXIST'.
Can you help me to learn more?
(Other definitions for exist that I've seen before include "Have objective reality" , "Live" , "Get a life!" , "Have being or continue living" , "Scrape by" .)