Smack climbing insect (4)
I believe the answer is:
gnat
'insect' is the definition.
(gnat is a kind of insect)
'smack climbing' is the wordplay.
'smack' becomes 'tang' ('tang' can be a synonym of 'smack').
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'tang' backwards is 'GNAT'.
(Other definitions for gnat that I've seen before include "Something that bites" , "mozzie" , "Tiny biting insect" , "insect's" , "Small fly that bites" .)