Occasionally steamy Irish brew (3)
I believe the answer is:
'irish brew' is the definition.
(Irish dialect term for tea)
'occasionally steamy' is the wordplay.
'occasionally' means one should take alternating letters.
The alternate letters of 'steamy' are 'TAY'.
(Other definitions for tay that I've seen before include "Scottish river in Katayama?" , "Scotland's longest river" , "Longest river of Scotland" , "Longest Scottish river" , "Scottish river and firth" .)