Poisonous reptile disconcerts nude no end (11)
I believe the answer is:
rattlesnake
'poisonous reptile' is the definition.
'disconcerts nude no end' is the wordplay.
'disconcerts' becomes 'rattles' (to rattle is to upset or disturb someone).
'nude' becomes 'naked' (synonyms).
'no end' means to remove the last letter.
'naked' with its final letter taken off is 'nake'.
'rattles'+'nake'='RATTLESNAKE'
(Other definitions for rattlesnake that I've seen before include "Poisonous (and noisy!) reptile" , "Potentially lethal reptile" , "Venomous creature" , "Virulent type" , "Sidewinder" .)