Austere Romeo in mess (5)
I believe the answer is:
harsh
'austere' is the definition.
(thesaurus)
'romeo in mess' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'mess' becomes 'hash' (I've seen this before).
'r' going inside 'hash' is 'HARSH'.
(Other definitions for harsh that I've seen before include "Strict, unfair" , "Not relenting" , "Abrasive, rough or unkind" , "Severe and disagreeable" , "Severe - unkind" .)