Cases of fortified wine fellow starts to unceremoniously swill after evening meal (12)

'cases' is the definition.
(portmanteau is a kind of case)

'fortified wine fellow starts to unceremoniously swill after evening meal' is the wordplay.
'fortified wine' becomes 'port' (port is a type of fortified wine).
'fellow' becomes 'man' (fellow can mean a man).
'starts to' suggests taking the first letters.
'after' means one lot of letters go next to another.
'evening meal' becomes 'tea' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The first letters of 'unceremoniously swill' is 'us'.
'us' after 'tea' is 'teaus'.
'port'+'man'+'teaus'='PORTMANTEAUS'

'of' is the link.

Can you help me to learn more?