When arches collapse, you need a drink (6)
I believe the answer is:
chaser
'a drink' is the definition.
(I've seen this before)
'when arches collapse' is the wordplay.
'arches' anagrams to 'CHASER'.
However, I'm not clear how this is indicated.
'you need' is the link.
Can you help me to learn more?
(Other definitions for chaser that I've seen before include "Milder drink taken after a stronger one" , "One hunting" , "engraver" , "Beer after whisky - follow that!" , "Drink of beer after spirit" .)