A note in support of strong ruler (9)
I believe the answer is:
potentate
'ruler' is the definition.
(potentate is a kind of ruler)
'a note in support of strong' is the wordplay.
'a note' becomes 'ate' (I've seen this in other clues**).
'in support of' means one lot of letters go next to another (I've seen this in other clues).
'strong' becomes 'potent' ('potent' can be similar in meaning to 'strong').
'ate' after 'potent' is 'POTENTATE'.
(Other definitions for potentate that I've seen before include "A monarch or ruler" , "Powerful autocratic ruler" , "Mogul" , "authority" .)