Locks books inside? That's bluster! (3,3)
I believe the answer is:
hot air
'bluster' is the definition.
(I've seen this in another clue)
'locks books inside?' is the wordplay.
'locks' becomes 'hair' (locks can mean hair).
'books' becomes 'OT' (Old Testament).
'inside?' means one lot of letters goes inside another.
'hair' placed around 'ot' is 'HOT-AIR'.
'that's' is the link.
(Other definitions for hot air that I've seen before include "Guff" , "is essential before the balloon goes up" , "Bluster" , "Bombast or balloon filler" , "Bull" .)