Resist old Irishman (5)
I believe the answer is:
bucko
'irishman' is the definition.
(I have seen 'Young Irishman' mean 'bucko' so perhaps 'irishman' could also mean 'bucko')
'resist old' is the wordplay.
'resist' becomes 'buck' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'buck'+'o'='BUCKO'
(Other definitions for bucko that I've seen before include "One swaggering" , "Young lad" , "Young Irishman" , "Lively young Irish fellow" , "Fellow swaggering" .)