Sea-dweller, having lost head, going off course (6)
I believe the answer is:
erring
'sea-dweller having lost head going off course' is the definition.
I can't judge whether this defines the answer.
'sea-dweller having lost head going off' is the wordplay.
'sea dweller' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'having lost' is a deletion indicator.
'head' becomes 'nut' (informal term for somebody's head).
'going off' becomes 'turning' (eg rotting food turns or goes off).
'turning' with 'nut' taken out is 'ring'.
'er'+'ring'='ERRING'
Can you help me to learn more?
(Other definitions for erring that I've seen before include "Being incorrect" , "Sinful" , "Making a mistake" , "taking wrong direction" , "Going wrong" .)