Smack fly that’s up (4)
I believe the answer is:
tang
'smack' is the definition.
(synonyms)
'fly that's up' is the wordplay.
'fly' becomes 'gnat' (I've seen this before).
'that's up' is a reversal indicator (in down clue: letters go upwards).
'gnat' back-to-front is 'TANG'.
(Other definitions for tang that I've seen before include "Coarse seaweed" , "Tart spiciness" , "Bite" , "The biting flavour of seaweed" , "Distinctive taste" .)