Might take your old form in a sneaky manner (8)
I believe the answer is:
stealthy
'might' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives sometimes define each other.
'take your old form in a sneaky manner' is the wordplay.
I cannot really see how this works, but
'take' could be 'steal' (stealing is a kind of taking) and 'steal' is found within the answer.
'your' could be 'thy' ('thy' historically meant 'your') and 'thy' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stealthy that I've seen before include "Unobtrusive" , "Covert - clandestine" , "Secretive, silent" , "Acting in quiet or secret way" , "'Sneaky, clandestine (8)'" .)