Nothing in new design of nicest part (7)
I believe the answer is:
section
'part' is the definition.
(synonyms)
'nothing in new design of nicest' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'in' means one lot of letters goes inside another.
'new design of' is an anagram indicator.
'nicest' is an anagram of 'sectin'.
'o' inserted inside 'sectin' is 'SECTION'.
(Other definitions for section that I've seen before include "Part, subdivision" , "Element" , "Notices (anag)" , "Division, portion" , "unit" .)