Stop first, joining a coffee expert (7)
I believe the answer is:
barista
'coffee expert' is the definition.
(I've seen this before)
'stop first joining a' is the wordplay.
'stop' becomes 'bar' (bar can mean to stop or prevent).
'first' becomes 'ist' (ist resembles 1st).
'joining' means one lot of letters go next to another.
'bar'+'ist'+'a'='BARISTA'
(Other definitions for barista that I've seen before include "person serving drinks" , "Starbucks employee" , "Coffee expert" , "coffee-shop employee" , "Arabist (anag)" .)