... last of port, ale and cider, shifting the equivalent of 2.2 gallons! (9)

'equivalent of 2 2 gallons' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'last of port ale and cider shifting' is the wordplay.
'last of' indicates one should take the final letters.
'and' is a charade indicator (letters next to each other).
'shifting' indicates an anagram.
The final letter of 'port' is 't'.
't'+'ale'='tale'
'tale' put after 'cider' is 'cidertale'.
'cidertale' with letters rearranged gives 'DECALITRE'.

'the' is the link.

Can you help me to learn more?