Book sizes up snake wound (6)
I believe the answer is:
psalms
'book sizes' is the definition.
'psalms' can be an answer for 'book' (I've seen this before). I'm not certain of the 'sizes' bit.
'up snake wound' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'snake' becomes 'asp' (asp is a kind of snake).
'wound' becomes 'lms' (I can't explain this - if you can you should give a lot more credence to this answer).
'asp' backwards is 'psa'.
'psa'+'lms'='PSALMS'
Can you help me to learn more?
(Other definitions for psalms that I've seen before include "OT book" , "David's hymns" , "Holy songs" , "Biblical songs" , "They're heard in church" .)