Men meeting one unknown, then another, beast (4)
I believe the answer is:
oryx
'beast' is the definition.
(I've seen this before)
'men meeting one unknown then another' is the wordplay.
'men' becomes 'OR' (military abbreviation for other ranks).
'meeting' says to put letters next to each other.
'one unknown' becomes 'y' (algebra).
'then another' becomes 'x' (I can't explain this - if you can you should believe this answer much more).
'or'+'y'+'x'='ORYX'
(Other definitions for oryx that I've seen before include "Animal" , "horny beast" , "African native" , "creature" , "Large African antelope with long straight horns" .)