Just a little biting insect inside tin (7)
I believe the answer is:
smidgen
'just a little' is the definition.
'biting insect inside tin' is the wordplay.
'biting insect' becomes 'midge' (I've seen this before).
'inside' is an insertion indicator.
'tin' becomes 'sn' (Sn is the chemical symbol for tin).
'midge' placed inside 'sn' is 'SMIDGEN'.
(Other definitions for smidgen that I've seen before include "Very little" , "Tiny amount" , "bit!" , "A very small amount (7)." , "A tad" .)