When arches collapse, you need a drink (6)
I believe the answer is:
chaser
'a drink' is the definition.
(as in a shot and chaser)
'when arches collapse you' is the wordplay.
'arches' anagrams to 'CHASER'.
However, I'm not clear how the anagram is indicated.
'need' acts as a 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" .)