Open book to appear threatening (5)
I believe the answer is:
bloom
'open' is the definition.
(I've seen this before)
'book to appear threatening' is the wordplay.
I cannot quite understand how this works, but
'book' could be 'b' (abbreviation for book) and 'b' is found in the answer.
'appear' could be 'loom' (looming is a kind of appearing) and 'loom' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bloom that I've seen before include "Produce flowers" , "look good" , "Flourish" , "Do very well" , "Flush" .)