Rainstorm in Ulster county, on the rocks some say? (8)
I believe the answer is:
downpour
'rainstorm' is the definition.
(thesaurus)
'ulster county on the rocks some say?' is the wordplay.
'ulster county' becomes 'down' (County Down in Northern Ireland).
'on the rocks' becomes 'poor'.
'some say?' indicates a 'sounds like' (homophone) clue (I've seen 'say' mean this).
'poor' sounds like 'pour'.
'down'+'pour'='DOWNPOUR'
'in' acts as a link.
(Other definitions for downpour that I've seen before include "Torrent" , "cats and dogs?" , "Heavy shower" , "falling water" , "Big fall makes" .)