Persistent cautions deployed involving ecstasy (9)
I believe the answer is:
tenacious
'persistent' is the definition.
(I know that persistent can be written as tenacious)
'cautions deployed involving ecstasy' is the wordplay.
'deployed' indicates an anagram.
'involving' is an insertion indicator.
'ecstasy' becomes 'E' (informal term for the drug).
'cautions' with letters rearranged gives 'tnacious'.
'tnacious' going around 'e' is 'TENACIOUS'.
(Other definitions for tenacious that I've seen before include "Persistent in holding on" , "Keeping a firm hold" , "Determined" , "Holding fast" , "refusing to give up" .)