Monarch at home in poetic isle (4)
I believe the answer is:
erin
'poetic isle' is the definition.
(poetic name for Ireland)
'monarch at home' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina, British Queen).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'er'+'in'='ERIN'
'in' acts as a link.
(Other definitions for erin that I've seen before include "Land" , "'Come back to ...' (emigrant song)" , "Literary term for Ireland" , "country" , "The Emerald Isle, in poetry" .)