State Queen's child with German grandparent ultimately can be fighter in war (13)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

MESS (state, specifically a bad state, as in “look at the state you’re in!”) + ER’S (the late Queen’s) + CH (abbreviation for child) + MIT (“with” in German) + last letter (ultimately) of [grandparen] T . WW2-era fighter aircraft made by the German company of the same name.