Exploit involving record day with unknown going through (8)

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

Exploit involving record day with unknown going through [EP (=record, i.e. extended-play) + D (=day) + Y (=unknown, in algebra)] in DEED (=exploit); deep-dyed means through-and-through, dyed-in-the-wool complete, as a deep-dyed Tory or a deep-dyed villain