I'm doubtful magistrate has secured vault (4)
I believe the answer is:
jump
'vault' is the definition.
(vault can mean to jump)
'i'm doubtful magistrate has secured' is the wordplay.
I cannot quite see how this works, but
'doubtful' could be 'um' (saying 'um' communicates doubt) and 'um' is found within the answer.
'magistrate' could be 'jp' and 'jp' is located in the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for jump that I've seen before include "Sudden large increase" , "Vault, bound" , "Attack" , "Suddenly rise" , "Suddenly increase" .)