Certain notes, or one little letter (7)
I believe the answer is:
minutes
'certain notes' is the definition.
'minutes' can be an answer for 'notes' (minute is a kind of note). I'm not certain of the 'certain' bit.
'one little letter' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'letter' could be 'nu' (nu is a kind of letter) and 'nu' is found within the answer.
This explanation may well be incorrect...
'or' is the link.
Can you help me to learn more?
(Other definitions for minutes that I've seen before include "Records" , "Record of a meeting" , "summary" , "Units of time" , "Written account of a meeting" .)