Purchase endless beer, drinking constantly (8)
I believe the answer is:
leverage
'purchase' is the definition.
(thesaurus)
'endless beer drinking constantly' is the wordplay.
'endless' means to remove the last letter.
'beer' becomes 'lager' (lager is a kind of beer).
'drinking' means one lot of letters goes inside another.
'constantly' becomes 'ever' (I've seen this before).
'lager' with its final letter removed is 'lage'.
'lage' placed around 'ever' is 'LEVERAGE'.
(Other definitions for leverage that I've seen before include "Mechanical advantage; influence" , "Power to act effectively" , "Power to influence" , "Power of achievement; (US) gearing" , "What crowbar provides" .)