Fish — there's unknown number in school? (5)
I believe the answer is:
tench
'fish' is the definition.
(tench is a kind of fish)
'unknown number in school?' is the wordplay.
'unknown number' becomes 'n'.
'in' indicates putting letters inside.
'school?' becomes 'tech' (tech is a kind of school).
'n' inserted within 'tech' is 'TENCH'.
'there's' acts as a link.
(Other definitions for tench that I've seen before include "something fishy" , "Swimmer" , "Fish of the carp family" , "catch" , "One in school" .)