Taking joint, he's addled, not entirely able to control matters ... (2,3,6)
I believe the answer is:
in the saddle
'able to control matters' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'taking joint he's addled not entirely' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'IN THE SADDLE' is hidden in the letters of 'taking joint hes addled'.
Can you help me to learn more?
(Other definitions for in the saddle that I've seen before include "Rider's position" , "as a Mountie?" , "On horseback (and in control!)" , "Exercising control" .)