Persistent cautions with police at last mobilised (9)
I believe the answer is:
tenacious
'persistent' is the definition.
(I know that persistent can be written as tenacious)
'cautions with police at last mobilised' is the wordplay.
'with' is a charade indicator (letters next to each other).
'at last' says to take the final letters.
'mobilised' indicates anagramming the letters (I've seen this in other clues).
The last letter of 'police' is 'e'.
'cautions' after 'e' is 'ecautions'.
'ecautions' with letters rearranged gives 'TENACIOUS'.
(Other definitions for tenacious that I've seen before include "Keeping a firm hold" , "Determined" , "Holding or sticking firmly" , "Persistent in holding on" , "refusing to give up" .)