Trainee's pal erratically holding up sink (7)
I believe the answer is:
relapse
'sink' is the definition.
(I've seen this before)
'trainee's pal erratically holding up' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'espaler' can be found hidden inside 'trainees pal erratically'.
'espaler' back-to-front is 'RELAPSE'.
(Other definitions for relapse that I've seen before include "Unexpected deterioration" , "Degenerate like pleaser" , "setback in health terms" , "Sink back into illness" , "Take a turn for the worse" .)