Nude worried by start of Rubenesque wrinkle, causing stress (9)
I believe the answer is:
underline
'stress' is the definition.
(underlining is a kind of stressing)
'nude worried by start of rubenesque wrinkle' is the wordplay.
'worried' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'start of' indicates taking the first letters.
'wrinkle' becomes 'line' ('line' can be a synonym of 'wrinkle'**).
The initial letter of 'rubenesque' is 'r'.
'nude' with letters rearranged gives 'unde'.
'unde'+'r'+'line'='UNDERLINE'
'causing' acts as a link.
(Other definitions for underline that I've seen before include "Call attention to - highlight" , "Method of showing emphasis in writing" , "Reinforce" , "Emphasise, stress importance" , "To show emphasis in writing" .)