Chap about to get cross (3)
I believe the answer is:
rex
'chap' is the definition.
'about to get cross' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to get' says to put letters next to each other.
'cross' becomes 'x' (X is cross-shaped).
're'+'x'='REX'
(Other definitions for rex that I've seen before include "Dog's name; king" , "Part of official title of a king" , "King (L.)" , "One in the palace" , "Oedipus - -" .)