I gave 'AIL'. using the verb 'to trouble' would give you 'AIL' and knocking the 'F' from 'Fail' (collapse) would give you the same. But I appreciate your concern.
I asked this earlier as I was bothered too.....the answer I got was also AIL from (F)AIL, so that's what I put in.
We shall find out soon I guess, but I really don't like the clue.