The sailor struck out, as is the custom (5)
I believe the answer is:
habit
'the custom' is the definition.
(habit is a kind of custom)
'the sailor struck out' is the wordplay.
'the sailor' becomes 'ab' (abbreviation for able seaman).
'struck' becomes 'hit' (I've seen this in another clue).
'out' is an insertion indicator.
'ab' placed inside 'hit' is 'HABIT'.
'as is' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Riding garment" , "Custom - costume" , "Custom or established practice" , "Monk's robe or repetitive action" , "Custom, nun's wear" .)