Purchase about fifty pounds of beef (5)
I believe the answer is:
bully
'beef' is the definition.
(bully beef is a kind of beef)
'purchase about fifty pounds' is the wordplay.
'purchase' becomes 'buy' (synonyms).
'about' indicates putting letters inside.
'fifty' becomes 'L' (Roman numeral).
'pounds' becomes 'L' (L can represent the monetary pound).
'l'+'l'='ll'
'buy' enclosing 'll' is 'BULLY'.
'of' is the link.
(Other definitions for bully that I've seen before include "Flashman, say" , "Browbeater" , "excellent" , "Playground thug" , "Well done" .)