Nudes run to get key quick (6)
I believe the answer is:
sudden
'quick' is the definition.
(similar in meaning)
'nudes run to get key' is the wordplay.
'run' indicates anagramming the letters.
'to get' is an insertion indicator.
'key' becomes 'd' (musical key).
'nudes' anagrammed gives 'suden'.
'suden' enclosing 'd' is 'SUDDEN'.
(Other definitions for sudden that I've seen before include "Immediate and surprising" , "Rapid and unexpected" , "Occurring unexpectedly quickly" , "Happening quickly and without warning" , "Swift" .)