It's customary for a seafarer to strike out (5)
I believe the answer is:
habit
'it's customary' is the definition.
I can't tell whether this definition defines the answer.
'seafarer to strike out' is the wordplay.
'seafarer' becomes 'ab' (I can't justify this - if you can you should give a lot more credence to this answer).
'to strike' becomes 'hit' (hitting is a kind of striking).
'out' is an insertion indicator.
'ab' placed within 'hit' is 'HABIT'.
'for a' is the link.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Custom, nun's wear" , "It's worn in order" , "addictive practice" , "Settled tendency" , "bent" .)