Irritate chap, forgetting his name? (3)
I believe the answer is:
get
'irritate' is the definition.
(I've seen this in another clue)
'chap forgetting his name?' is the wordplay.
'chap' becomes 'gent' (both can mean a man).
'forgetting his' is a deletion indicator.
'name?' becomes 'n' (common abbreviation e.g. on forms).
'gent' with 'n' removed is 'GET'.
(Other definitions for get that I've seen before include "Procure from vegetables" , "Receive; obtain" , "Understand" , "Acquire" , "See 18" .)