Torment one on street wearing costume (8)
I believe the answer is:
distress
'torment' is the definition.
(thesaurus)
'one on street wearing costume' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on street' becomes 'st' (I've seen this in other clues).
'wearing' indicates putting letters inside.
'costume' becomes 'dress' (I've seen this before).
'i'+'st'='ist'
'ist' inserted into 'dress' is 'DISTRESS'.
(Other definitions for distress that I've seen before include "SOS is such a signal" , "Cause mental pain" , "Sadden" , "Anguish - trouble - danger" , "Extreme anxiety or sorrow" .)