Nothing to fasten? That's what you think! (7)
I believe the answer is:
opinion
'that's what you think' is the definition.
(I've seen this in another clue)
'nothing to fasten?' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'to fasten?' becomes 'pinion' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'o'+'pinion'='OPINION'
Can you help me to learn more?
(Other definitions for opinion that I've seen before include "Estimation" , "Conviction - point of view" , "View, barrister's advice" , "Personal belief or judgment" , "Assessment" .)