One having party back in holiday island (4)
I believe the answer is:
bali
'holiday island' is the definition.
'one having party back' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'having' means one lot of letters go next to another.
'party' becomes 'lab' (abbreviation for Labour).
'back' says the letters should be written in reverse.
'i'+'lab'='ilab'
'ilab' in reverse letter order is 'BALI'.
'in' is the link.
(Other definitions for bali that I've seen before include "Island in Indonesia" , "Indonesian island, east of Java" , "Bail out of the island" , "Holiday island to the east of Java" , "mountainous island" .)