Henry is working to block agent, backing mind-control (8)
I believe the answer is:
hypnosis
'mind-control' is the definition.
(means of influencing someone's mind)
'henry is working to block' is the wordplay.
'henry' becomes 'hypnos' (I can't explain this - if you can you should believe this answer much more).
'working' indicates an anagram.
'to block' indicates putting letters inside (block can mean to fill a hole in something).
'is' with letters rearranged gives 'si'.
'hypnos' enclosing 'si' is 'HYPNOSIS'.
'agent backing' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for hypnosis that I've seen before include "Means of inducing trance" , "Induced state of trance" , "Sleeplike state in which mind responds to suggestions" , "Suggestive technique" , "Mesmerism" .)