They’re customary when a seaman strikes out (6)
I believe the answer is:
habits
'they're customary' is the definition.
The definition suggests an adverb but the answer is not.
'a seaman strikes out' is the wordplay.
'a seaman' becomes 'ab'.
'strikes' becomes 'hits' ('hit' can be a synonym of 'strike').
'out' indicates putting letters inside.
'ab' placed into 'hits' is 'HABITS'.
'when' is the link.
Can you help me to learn more?
(Other definitions for habits that I've seen before include "Customs in nun's clothing?" , "Monks' and nuns' clothing" , "Customs, wonts" , "Uniforms of religious orders" , "Customs or robes" .)