Commons' terseness conceals fiendish things (8)
I believe the answer is:
monsters
'fiendish things' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.
'commons' terseness conceals' is the wordplay.
'conceals' indicates the answer is hidden within the clue.
'MONSTERS' is hidden in the letters of 'commons terseness'.
Can you help me to learn more?
(Other definitions for monsters that I've seen before include "'Imaginary creatures, large and wicked (8)'" , "Imaginary frightening creatures" , "Behemoths, fiends" , "Large and frightening creatures" , "Large imaginary creatures, usually wicked" .)