Where, informally, you need a bottle opener for a drink (4)
I believe the answer is:
brum
'where informally' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'a bottle opener for a drink' is the wordplay.
'opener' says to take the initial letters.
'for' is a charade indicator (letters next to each other).
'a drink' becomes 'rum' (rum is a kind of drink).
The initial letter of 'bottle' is 'b'.
'b'+'rum'='BRUM'
'you need' acts as a link.
Can you help me to learn more?
(Other definitions for brum that I've seen before include "Birmingham's nickname" , "Birmingham (abbrev.)" , "Birmingham (informal)" , "Birmingham (abbr)" .)