Flashing beam - the middle of three? (5)

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

Anagram (flashing?) of BEAM , then the middle letter of [th] R [ee]. Clue-as-definition (&lit): amber is the middle light in a traffic-light sequence, and in some cases it’s a flashing light rather than a steady one.