Not a good sailor (6)
I believe the answer is:
pirate
'sailor' is the definition.
(I've seen this before)
'not a good' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'good' could be 'pi' (old-fashioned term for pious) and 'pi' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for pirate that I've seen before include "Seaborne robber" , "Illegally-copied (version)" , "plagiarist" , "Criminal on the high seas" , "Hook", for example" .)