Very small gnat with top of tail showing (6)
I believe the answer is:
midget
'very small' is the definition.
(I've seen this before)
'gnat with top of tail showing' is the wordplay.
'gnat' becomes 'midge' (midge is a kind of gnat).
'with' says to put letters next to each other.
'top of tail showing' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'midge'+'t'='MIDGET'
Can you help me to learn more?
(Other definitions for midget that I've seen before include "Little person" , "Manikin" , "Happy for one" , "Dwarf" , "Tiny" .)