Best line to a southern port (5)
I believe the answer is:
ideal
'best' is the definition.
(similar in meaning)
'line to a southern port' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'to a' is a charade indicator (letters next to each other) (I've seen this in other clues).
'southern port' becomes 'idea' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'l' after 'idea' is 'IDEAL'.
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "Something that one hopes to attain" , "faultless" , "without a flaw" , "'Perfect, model (5)'" , "Perfect for the purpose" .)