Penthouse for one visiting oligarch's outside Irish town (5)
I believe the answer is:
omagh
'irish town' is the definition.
(town in County Tyrone)
'penthouse for one visiting oligarch's outside' is the wordplay.
'penthouse for' becomes 'mag' (I can't explain this - if you can you should give a lot more credence to this answer).
'one visiting' means one lot of letters goes inside another.
'outside' means to remove the middle letters.
'oligarch' with its middle removed is 'oh'.
'mag' put into 'oh' is 'OMAGH'.
Can you help me to learn more?
(Other definitions for omagh that I've seen before include "somewhere in Northern Ireland" , "Main town of County Tyrone" , "Ulster town" , "N. Irish town, district" , "NI town" .)