Such neat design becoming immodest (8)
I believe the answer is:
unchaste
'immodest' is the definition.
(thesaurus)
'such neat design' is the wordplay.
'design' indicates anagramming the letters.
'such'+'neat'='suchneat'
'suchneat' anagrammed gives 'UNCHASTE'.
'becoming' is the link.
(Other definitions for unchaste that I've seen before include "Sexually immodest" , "Immoral, wanton" , "No longer pure" , "very naughty" , "Lascivious" .)