Northern swimmer Jack sent round extract from Moby-Dick? (6)
I believe the answer is:
baleen
'extract from moby-dick?' is the definition.
I can't tell whether this defines the answer.
'northern swimmer jack sent round' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'swimmer' becomes 'eel' (eel is a kind of fish).
'jack' becomes 'ab' (both can mean a sailor).
'sent round' shows that the letters should be reversed in order.
'n'+'eel'+'ab'='neelab'
'neelab' written backwards gives 'BALEEN'.
Can you help me to learn more?
(Another definition for baleen that I've seen is " Enable (anag) - whalebone".)