An irritable “That’s how old port should look” (6)
I believe the answer is:
crusty
'an irritable' is the definition.
(thesaurus)
'how old port should look' is the wordplay.
I cannot really see how this works, but
'old' could be 'rusty' (similar in meaning) and 'rusty' is found in the answer.
A single letter 'c' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for crusty that I've seen before include "Cross or harsh, like outer layer of loaf" , "Short" , "Ill-humoured" , "Sour and brusque, like outer part of bread" , "Curtsy (anag.)" .)