Start books describing person’s occasional reflection (5)
I believe the answer is:
onset
'start' is the definition.
(both can mean the beginning of something)
'books describing person's occasional reflection' is the wordplay.
'books' becomes 'OT' (Old Testament).
'describing' means one lot of letters goes inside another (describe can mean to form the outline of).
'person's occasional' becomes 'esn' (I can't explain this - if you can you should give a lot more credence to this answer).
'reflection' shows that the letters should be reversed in order.
'esn' backwards is 'nse'.
'ot' going around 'nse' is 'ONSET'.
Can you help me to learn more?
(Other definitions for onset that I've seen before include "Storming" , "Assault; beginning" , "Outbreak" , "Beginning or early stages" , "Advent" .)