US on move turning deadly (8)
I believe the answer is:
venomous
'deadly' is the definition.
('venomous' can be a synonym of 'deadly')
'us on move turning' is the wordplay.
'turning' is an anagram indicator.
'us'+'on'+'move'='usonmove'
'usonmove' with letters rearranged gives 'VENOMOUS'.
(Other definitions for venomous that I've seen before include "Poisonous like many snakes" , "Toxic - spiteful" , "(Of snake) injecting poison" , "acid" , "Extremely poisonous" .)