A drinks dispenser to retain on block (7)

'a drinks dispenser' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'retain on block' is the wordplay.
'retain' becomes 'keep' (keeping is a kind of retaining).
'on' says to put letters next to each other.
'block' becomes 'bar' (block can mean to ban or bar).
'keep' put after 'bar' is 'BARKEEP'.

'to' is the link.

Can you help me to learn more?