Where ocean liners travel, mostly (2,3,4)
I believe the answer is:
in the main
'mostly' is the definition.
(both can mean generally)
'where ocean liners travel' is the wordplay.
I cannot really understand how this works, but
'ocean' could be 'main' (synonyms) and 'main' is found within the answer.
The remaining letters 'inthe' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for in the main that I've seen before include "usually" , "On the whole" , "Where ships are" , "All things considered" , "Almost entirely" .)