Tiny binder supposedly at first getting lost under old notepaper (5)
I believe the answer is:
gluon
'tiny binder supposedly' is the definition.
(hypothetical particle which binds others together)
'first getting lost under old notepaper' is the wordplay.
'first' says to take the initial letters.
The initial letters of 'getting lost under old notepaper' is 'GLUON'.
'at' acts as a link.
(Other definitions for gluon that I've seen before include "23 5A [ELEMENTARY PARTICLE]" , "Hypothetical particle" , "Quark-binding particle" .)