Rum does when knocked back, so a port is required (6)
I believe the answer is:
odessa
'a port is required' is the definition.
I can't tell whether this defines the answer.
'rum does when knocked back' is the wordplay.
'rum' indicates anagramming the letters (rum can mean strange or unusual).
'when' becomes 'as'.
'knocked back' says the letters should be written backwards.
'as' back-to-front is 'sa'.
'does' with letters rearranged gives 'odes'.
'odes'+'sa'='ODESSA'
'so' acts as a link.
Can you help me to learn more?
(Other definitions for odessa that I've seen before include "Ukrainian Black Sea port" , "Port city on the Black Sea" , "Ukrainian naval base" , "Seaport" , "Ukrainian location" .)