Claw that's attached locked on nicely, initially (5)
I believe the answer is:
talon
'claw' is the definition.
(I know that talon is a type of claw)
'that's attached locked on nicely initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'thats attached locked on nicely' is 'TALON'.
(Other definitions for talon that I've seen before include "one that grips" , "E.g. hawk's claw" , "a gripping one" , "Tonal (anag.)" , "Eg, an eagle's claw" .)