Horrible tailless rodent suppresses wicked snort (9)
I believe the answer is:
monstrous
'horrible' is the definition.
(thesaurus)
'tailless rodent suppresses wicked snort' is the wordplay.
'tailless rodent' becomes 'mous' (I can't justify this - if you can you should give a lot more credence to this answer).
'suppresses' is an insertion indicator.
'wicked' indicates anagramming the letters.
'snort' with letters rearranged gives 'nstro'.
'mous' going around 'nstro' is 'MONSTROUS'.
Can you help me to learn more?
(Other definitions for monstrous that I've seen before include "Outrageously wrong" , "really good!" , "Quite outrageous" , "Shockingly brutal or cruel" , "Extraordinarily large" .)