A blowout for a seafarer, as a rule? (5)
I believe the answer is:
habit
'a rule?' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Breaking this is hard" , "Custom; clothing" , "Custom; riding dress" , "Monk's garb" , "Settled tendency" .)