Trooper is last in company to turn 18 (10)

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

So clue 18 provides the definition. For the wordplay, it helps if you realise there’s no A in the anagram (to turn) of TROOPER IS plus the Y from the end of companY. I didn’t. On edit: great embarrassment: I can’t even correct it properly!