Degenerate put up trellis after one departs (7)

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

ESPAL [i] ER (a trellis on which plants are trained to grow up a wall), reversed (put up = upwards in a down clue), without the I (one in Roman numerals). Degenerate = relapse = return to a previous poor state.