Insect crossing black sticky stuff is slowing down (9)
I believe the answer is:
retardant
'is slowing down' is the definition.
(retardant can mean tending to slow down)
'insect crossing black sticky stuff' is the wordplay.
'insect' becomes 'red ant' (I've seen this in another clue).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'black sticky stuff' becomes 'tar' (I've seen this in another clue).
'redant' going around 'tar' is 'RETARDANT'.
(Other definitions for retardant that I've seen before include "Inhibiting substance" , "having slowing effect" , "It delays" , "Preventing, inhibiting" , "Substance suppressing fire" .)