Tiniest particle in balance (8)
I believe the answer is:
remotest
'tiniest' is the definition.
(eg the tiniest or most remote chance)
'particle in balance' is the wordplay.
'particle' becomes 'mote' (synonyms).
'in' is an insertion indicator.
'balance' becomes 'rest' (I've seen this before).
'mote' put into 'rest' is 'REMOTEST'.
(Other definitions for remotest that I've seen before include "Terse mot (anag)" , "Furthest away, most distant" .)