Number gets fastened inside (3)
I believe the answer is:
ten
'number' is the definition.
(ten is a kind of number)
'gets fastened inside' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'TEN' is hidden within 'gets fastened'.
(Other definitions for ten that I've seen before include "Five times two" , "X-WORD" , "Base of decimal system" , "Figure" , "Roman numeral X" .)