Shout about terrible trade in inhibitor (9)
I believe the answer is:
retardant
'inhibitor' is the definition.
(I've seen this in another clue)
'shout about terrible trade' is the wordplay.
'shout' becomes 'rant' (I've seen this in other clues).
'about' indicates putting letters inside.
'terrible' is an anagram indicator.
'trade' with letters rearranged gives 'etard'.
'rant' going around 'etard' is 'RETARDANT'.
'in' is the link.
(Other definitions for retardant that I've seen before include "It delays" , "Substance suppressing fire" , "Inhibiting substance" , "Preventing, inhibiting" , "having slowing effect" .)