Historic vessel — it twice turned to round a Northern cape (7)
I believe the answer is:
titanic
'historic vessel' is the definition.
'titanic' can be an answer for 'vessel' (I've seen this before). I'm not sure about the 'historic' bit.
'it twice turned to round a northern cape' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'twice turned' becomes 'inti' (I can't explain this - if you can you should give a lot more credence to this answer).
'to round' indicates putting letters inside.
'northern' shows that the letters should be reversed in order (in a down clue, letters go up or 'north').
'cape' becomes 'c' (abbreviation used on maps).
'inti' going around 'a' is 'inati'.
'inati' in reverse letter order is 'itani'.
't'+'itani'+'c'='TITANIC'
Can you help me to learn more?
(Other definitions for titanic that I've seen before include "Famously sunken ship" , "Gigantic like a famous doomed liner" , "In attic (anag.)" , "Twenty one [SHIP]" , "Exceptionally powerful" .)