What quartet may do: pub-crawl, imbibing ales, expending energy (6-4)

'what quartet may do' is the definition.
(as in a barber-shop quartet)

'pub-crawl imbibing ales expending energy' is the wordplay.
'pub-crawl' becomes 'bar hop' (synonyms).
'imbibing' is an insertion indicator.
'ales' becomes 'beers' (I've seen this before).
'expending' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'beers' with 'e' removed is 'bers'.
'barhop' placed around 'bers' is 'BARBER-SHOP'.