A little tin with fly in it (7)
I believe the answer is:
smidgen
'a little' is the definition.
(thesaurus)
'tin with fly in it' is the wordplay.
'tin' becomes 'sn' ('Sn' can be a synonym of 'tin').
'with fly' becomes 'midge' (I've seen this before. I am not sure about the 'with' bit.**).
'in it' indicates putting letters inside.
'sn' placed around 'midge' is 'SMIDGEN'.
Can you help me to learn more?
(Other definitions for smidgen that I've seen before include "Iota" , "A very small amount (7)." , "A tad" , "Scarcely detectable amount" , "Dab" .)