Prophet dashing first set of tablets? (5)

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

We had difficulty parsing this, but with some help from a friend, we concluded that it is a clue-as-definition: a MOS (prophet) missing or ‘dashing’ the first letter + E S (ecstasy tablets) – the ‘set’ is superfluous in the wordplay – not our favourite clue