Bottom half of barrel old cook emptied (7)

'bottom half' is the definition.
(each buttock is half of a person's bottom)

'barrel old cook emptied' is the wordplay.
'barrel' becomes 'butt' (butt is a kind of barrel).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'emptied' says to hollow out the word (remove centre letters).
'cook' with its middle removed is 'ck'.
'butt'+'o'+'ck'='BUTTOCK'

'of' is the link.