He's about to get cross (3)
I believe the answer is:
rex
'he's' is the definition.
'about to get cross' is the wordplay.
'about to get' becomes 're' (I've seen this in other clues).
'cross' becomes 'x' (an X is a cross).
're'+'x'='REX'
(Other definitions for rex that I've seen before include "Dog's name; king" , "Oedipus - -" , "One in the palace" , "King (L.)" , "King (Latin); cat breed" .)