Military group ready to intervene in turn for writing on the wall? (8)

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

[ RAF(abbrev. for the Royal Air Force, a group in the British military) + FIT(ready to act/having the ability to undertake a task promptly) ] containing ( to intervene in ) GO(one’s turn/a chance to do or use something that comes in turn).