To gatecrash home arrogant, ultimately, and impolite (7)
I believe the answer is:
intrude
'to gatecrash' is the definition.
('intrude' can be a synonym of 'gatecrash')
'home arrogant ultimately and impolite' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'ultimately' indicates one should take the final letters.
'and' is a charade indicator (letters next to each other).
'impolite' becomes 'rude' ('rude' can be similar in meaning to 'impolite').
The final letter of 'arrogant' is 't'.
'in'+'t'+'rude'='INTRUDE'
(Other definitions for intrude that I've seen before include "Encroach on others' privacy" , "Break in" , "Because of temperature, blue" , "Push in unwantedly" , "Poke one's nose in" .)