Sailor in theatrical success - thats just routine (5)
I believe the answer is:
habit
'just routine' is the definition.
'habit' can be an answer for 'routine' (I've seen this before). I'm not sure about the 'just' bit.
'sailor in theatrical success' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in' is an insertion indicator.
'theatrical success' becomes 'hit' (hit is a kind of success. I am not sure about the 'theatrical' bit.).
'ab' put into 'hit' is 'HABIT'.
'thats' acts as a link.
Can you help me to learn more?
(Other definitions for habit that I've seen before include "Dress - custom - practice" , "Custom or established practice" , "Addiction to a drug" , "bent" , "nun gets into" .)