Turn up first part of encyclopaedia, perhaps — it's very small (4)
I believe the answer is:
iota
'very small' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'turn up first part of encyclopaedia perhaps' is the wordplay.
'turn up' says the letters should be written in reverse.
'first part of encyclopaedia perhaps' becomes 'A to I' (first nine volumes of an encyclopaedia).
'atoi' reversed gives 'IOTA'.
'it's' is the link.
(Other definitions for iota that I've seen before include "Ninth letter of Greek alphabet, small amount" , "Tiny piece" , "The Greek i (4)" , "Greek letter - small quantity" , "The smallest possible amount, or the Greek i" .)