Explanatory material little encountered in essential stuff given to Academy (8)
I believe the answer is:
metadata
'explanatory material' is the definition.
(metadata is data which explains other data)
'little encountered in essential stuff given to academy' is the wordplay.
'little' becomes 'tad'.
'encountered in' is an insertion indicator.
'essential stuff' becomes 'meat' (the substance or gist of something).
'given to' means one lot of letters go next to another.
'academy' becomes 'a' (abbreviation for academy).
'tad' placed into 'meat' is 'metadat'.
'metadat'+'a'='METADATA'
(Another definition for metadata that I've seen is " computer storage information".)