Very small amount of tin trapping biting insect (7)
I believe the answer is:
smidgen
'very small amount' is the definition.
(I've seen this before)
'tin trapping biting insect' is the wordplay.
'tin' becomes 'sn' ('Sn' can be a synonym of 'tin').
'trapping' means one lot of letters goes inside another.
'biting insect' becomes 'midge' (I've seen this before).
'sn' placed around 'midge' is 'SMIDGEN'.
'of' is the link.
(Other definitions for smidgen that I've seen before include "Very little" , "bit!" , "Tiny part" , "Iota" , "Scarcely detectable amount" .)