They may form a strong attachment to a craft (9)
I believe the answer is:
barnacles
'they may form a strong' is the definition.
The definition suggests an adverb but the answer is not.
'attachment to a craft' is the wordplay.
'attachment' becomes 'barnacle' (I have seen 'underwater attachment' mean 'barnacle' so perhaps 'attachment' could also mean 'barnacle').
'to a craft' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'barnacle'+'s'='BARNACLES'
Can you help me to learn more?
(Other definitions for barnacles that I've seen before include "crusty lodgers" , "Geese" , "Balancers for shellfish that stick to hulls" .)