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' suggests the final letters.
The last 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" .)