Just swarm up (4)
I believe the answer is:
meet
'just' is the definition.
('meet' can be similar in meaning to 'just')
'swarm up' is the wordplay.
'swarm' becomes 'teem' ('teem' can be a synonym of 'swarm').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'teem' written backwards gives 'MEET'.
(Other definitions for meet that I've seen before include "Join" , "acceptable" , "Come upon" , "Converge" , "Satisfy (requirements)" .)