Branch is above unknown multitude of ants? (4)
I believe the answer is:
army
'multitude of ants?' is the definition.
(army can mean a group of ants)
'branch is above unknown' is the wordplay.
'branch' becomes 'arm' (synonyms).
'is above' says to put letters next to each other (in a down clue, some letters go above others).
'unknown' becomes 'y' (algebra).
'arm'+'y'='ARMY'
(Other definitions for army that I've seen before include "Vast number" , "Military land forces" , "Military unit" , "Land-fighting force" , "A great number (of soldiers?)" .)