Smack insect from below (4)
I believe the answer is:
tang
'smack' is the definition.
(I know that smack can be written as tang)
'insect from below' is the wordplay.
'insect' becomes 'gnat' (gnat is a kind of insect).
'from below' says the letters should be written in reverse (in a down clue, letters go up).
'gnat' reversed gives 'TANG'.
(Other definitions for tang that I've seen before include "characteristic quality" , "tastefulness" , "Pungency" , "Beat, with good" , "Strong taste or smell" .)