Clouds issue, at first in support of Belfast doctor, maybe (5)
I believe the answer is:
nimbi
'clouds' is the definition.
(nimbus is a kind of cloud)
'at first in support of belfast doctor maybe' is the wordplay.
'at' means one lot of letters go next to another.
'first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'in support of' is a charade indicator (letters next to each other) (in a down clue, some letters hold up others).
'belfast' becomes 'NI' (Belfast is the capital of Northern Ireland).
'doctor maybe' becomes 'MB' (Bachelor of Medicine).
'ni' next to 'mb' is 'nimb'.
'i' put after 'nimb' is 'NIMBI'.
'issue' is the link.
(Other definitions for nimbi that I've seen before include "Dark things" , "Rain-bearing clouds" , "Rain-clouds" , "They may surround the head" , "rings" .)