Powerful ruler in old marquee introduced to head (9)
I believe the answer is:
potentate
'powerful ruler' is the definition.
(I've seen this before)
'old marquee introduced to head' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'marquee' becomes 'tent' (I've seen this before).
'introduced to' indicates putting letters inside.
'head' becomes 'pate' (I've seen this before).
'o'+'tent'='otent'
'otent' placed inside 'pate' is 'POTENTATE'.
'in' is the link.
(Other definitions for potentate that I've seen before include "Mogul" , "authority" , "Powerful autocratic ruler" , "A monarch or ruler" .)