Precise reference that's given at start of lesson (7,3,5)
I believe the answer is:
chapter and verse
'precise reference' is the definition.
'given at start of lesson' is the wordplay.
I cannot really see how this works, but
'given' could be 'apt' (synonyms) and 'apt' is present in the answer.
'at' could be 'her' (I've seen this in other clues) and 'her' is located in the leftover letters.
This may be the basis of the clue (or it may be nonsense).
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for chapter and verse that I've seen before include "complete specification" , "authority" , "location of Biblical text" , "Full" , "in detail" .)