Most senior of the French embraced by English lieutenant (6)
I believe the answer is:
eldest
'most senior' is the definition.
(I've seen this before)
'of the french embraced by english lieutenant' is the wordplay.
'of the french' becomes 'des'.
'embraced by' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'lieutenant' becomes 'lt'.
'e'+'lt'='elt'
'des' inserted into 'elt' is 'ELDEST'.
(Other definitions for eldest that I've seen before include "Most senior of a group" , "Most advanced in age, led set (anag.)" , "The oldest of three or more" , "heir?" , "Such as Cain, say" .)