Maybe it's come off, but not put back (6)
I believe the answer is:
button
'maybe it's come off' is the definition.
I can't judge whether this defines the answer.
'but not put back' is the wordplay.
'put back' shows that the letters should be reversed in order.
'not' in reverse letter order is 'ton'.
'but'+'ton'='BUTTON'
Can you help me to learn more?
(Other definitions for button that I've seen before include "Push it or do it up!" , "Bright as a - -" , "Garment fastener" , "Worthless, they say" , "Fastener used on clothes etc" .)