The attachments of companion vessels (11)

I believe the answer is:

friendships

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

'the attachments' is the definition.
Although both the answer and definition are plural nouns, I can't understand how one could define the other.

'companion vessels' is the wordplay.
'companion' becomes 'friend' (I've seen this before).
'vessels' becomes 'ships' (ship is a kind of vessel).
'friend'+'ships'='FRIENDSHIPS'

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in The Irish News.

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