Purchase about fifty pounds of beef (5)
I believe the answer is:
bully
'beef' is the definition.
(bully beef is corned beef)
'purchase about fifty pounds' is the wordplay.
'purchase' becomes 'buy' (synonyms).
'about' means one lot of letters goes inside another.
'fifty' becomes 'l' (Roman numeral).
'pounds' becomes 'l' (abbreviation e.g. Lsd - pounds, shillings, pence).
'l'+'l'='ll'
'buy' placed around '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" .)