Very small amount of insect in tin (7)
I believe the answer is:
smidgen
'very small amount' is the definition.
(I've seen this before)
'insect in tin' is the wordplay.
'insect' becomes 'midge' (midge is a kind of insect).
'in' is an insertion indicator.
'tin' becomes 'Sn' (Sn is the chemical symbol for tin).
'midge' put within 'sn' is 'SMIDGEN'.
'of' acts as a link.
(Other definitions for smidgen that I've seen before include "Taste" , "A tad" , "Iota" , "A very small amount (7)." , "Tiny part" .)