Poet in with Romeo (5)
I believe the answer is:
homer
'poet' is the definition.
(I know that Homer is a poet)
'in with romeo' is the wordplay.
'in' becomes 'home' ('he's in' can mean 'he's home').
'with' is a charade indicator (letters next to each other).
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'home'+'r'='HOMER'
(Other definitions for homer that I've seen before include "Bart Simpson's father" , "Great Greek who wrote The Iliad" , "Greek poet - TV's Simpson" , "The classical poet" , "Great Greek author of The Iliad" .)