Strengthen means of controlling stallion? (9)
I believe the answer is:
reinforce
'strengthen' is the definition.
(reinforcing is a kind of strengthening)
'means of controlling stallion?' is the wordplay.
I cannot really understand how this works, but
'means' could be 'rein' (I have seen 'Means of control' mean 'rein' so perhaps 'means' could also mean 'rein') and 'rein' is present in the answer.
'of' could be 'o' and 'o' is found in the answer.
This explanation may well be incorrect...
(Other definitions for reinforce that I've seen before include "increase support" , "provide support" , "Give added strength to" , "Strengthen or support with confrerie" , "Buttress" .)