Fasten firmly (5)
I believe the answer is:
rivet
'fasten firmly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rivet that I've seen before include "Arrest" , "Bolt for metal plates" , "Engross attention" , "Metal pin; engross" , "Heavy fastening pin with head and hammered end" .)