Sort of gland your old French king ultimately had (7)
I believe the answer is:
thyroid
'sort of gland' is the definition.
'your old french king ultimately had' is the wordplay.
'your old' becomes 'thy' ('thy' historically meant 'your').
'french king' becomes 'roi' ('king' in French).
'ultimately' indicates one should take the final letters.
The final letter of 'had' is 'd'.
'thy'+'roi'+'d'='THYROID'
(Other definitions for thyroid that I've seen before include "Gland in the neck" , "Growth-regulating gland" , "Growth-controlling gland" .)