Reptile disconcerts duke leaving with nothing on (11)
I believe the answer is:
rattlesnake
'reptile' is the definition.
(type of snake)
'disconcerts duke leaving with nothing on' is the wordplay.
'disconcerts' becomes 'rattles' (to rattle is to upset or disturb someone).
'duke' becomes 'd' (abbreviation).
'leaving' is a deletion indicator.
'with nothing on' becomes 'naked' (I've seen this in another clue).
'naked' with 'd' taken away is 'nake'.
'rattles'+'nake'='RATTLESNAKE'
(Other definitions for rattlesnake that I've seen before include "Sidewinder" , "Heavy-bodied American pit viper" , "Venomous creature" , "Poisonous creature" , "Poisonous (and noisy!) reptile" .)