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' becomes 're' (I've seen this in other clues).
'get' means one lot of letters go next to another.
'cross' becomes 'x' (X is cross-shaped).
'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" .)