I must have short party up in tropical resort (4)
I believe the answer is:
bali
'tropical resort' is the definition.
(Bali is a resort island)
'i must have short party up' is the wordplay.
'must have' says to put letters next to each other.
'short party' becomes 'lab' (abbreviation for Labour, UK political party; 'short' indicates an abbreviation).
'up' is a reversal indicator (in down clue: letters go upwards).
'i'+'lab'='ilab'
'ilab' back-to-front is 'BALI'.
'in' acts as a link.
(Other definitions for bali that I've seen before include "Indonesian holiday destination" , "Indonesian island, capital Denpasar" , "Indonesian island, east of Java" , "Island in Indonesia" , "mountainous island" .)