Nudes - about 500 - change without warning (6)
I believe the answer is:
sudden
'without warning' is the definition.
(I've seen this in another clue)
'nudes about 500 change' is the wordplay.
'about' means one lot of letters go next to another.
'500' becomes 'D' (Roman numeral for 500).
'change' indicates anagramming the letters.
'nudes' put after 'd' is 'dnudes'.
'dnudes' with letters rearranged gives 'SUDDEN'.
(Other definitions for sudden that I've seen before include "Rapid and unexpected" , "Abrupt, unexpected" , "Swift" , "Done unexpectedly" , "Unexpected, without warning" .)