A fanatic but firm cruise-ship (4-5)

I believe the answer is:

hard-liner

I'm a little stuck... Click here to teach me more about this clue!

'a fanatic' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'firm cruise-ship' is the wordplay.
'firm' becomes 'hard' (both can mean durable or strong).
'cruise ship' becomes 'liner' (I've seen this before).
'hard'+'liner'='HARD-LINER'

'but' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app