Arch is rough cast on the outside (7)
I believe the answer is:
roguish
'arch' is the definition.
(thesaurus)
'is rough cast on the outside' is the wordplay.
'cast' is an anagram indicator (letters are dispersed or cast around).
'on the outside' means one lot of letters goes inside another.
'rough' is an anagram of 'roguh'.
'is' placed inside 'roguh' is 'ROGUISH'.
(Other definitions for roguish that I've seen before include "Mischievous; unprincipled" , "Coy" , "Playfully mischievous" , "Dishonest - mischievous" , "reprobate" .)