A snake round one's neck! (3)
I believe the answer is:
boa
'a snake' is the definition.
(boa constrictor, type of snake)
'round one's neck' is the wordplay.
'round' shows that the letters should be reversed in order.
'one' becomes 'a' (a thing is one thing).
'neck' becomes 'ob' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'ob'='aob'
'aob' reversed gives 'BOA'.
Can you help me to learn more?
(Other definitions for boa that I've seen before include "Fluffy scarf of feathers" , "Snake; fur" , "S American snake that crushes its prey" , "One covered in scales" , "Large snake provides a neck decoration" .)