Horrid thing to ruin repose, ultimately? (9)
I believe the answer is:
nightmare
'horrid thing to ruin repose ultimately?' is the definition.
I can't tell whether this definition defines the answer.
'horrid thing to ruin repose ultimately?' is the wordplay.
'horrid' indicates an anagram.
'to ruin' becomes 'mar' (I've seen this before).
'ultimately?' indicates one should take the final letters (I've seen 'ultimately' mean this).
The last letter of 'repose' is 'e'.
'thing' anagrammed gives 'night'.
'night'+'mar'+'e'='NIGHTMARE'
Can you help me to learn more?
(Other definitions for nightmare that I've seen before include "Horror" , "Scary fantasy while asleep" , "Horrible experience" , "dreadful experience" , "It makes one restless" .)