Firm married fellow to the German officer (9)
I believe the answer is:
commander
'officer' is the definition.
(I know that commander is a type of officer)
'firm married fellow to the german' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'married' becomes 'm'.
'fellow' becomes 'man' (fellow can mean a man).
'to the german' becomes 'der' ('the' in German).
'co'+'m'+'man'+'der'='COMMANDER'
(Other definitions for commander that I've seen before include "Naval officer" , "Officer in charge of a military unit" , "The one who gives the orders" , "One hoping to be captain" , "Chief" .)