Climbing trained fruit-tree I abandoned shows return of disease (7)
I believe the answer is:
relapse
'return of disease' is the definition.
'climbing trained fruit-tree i abandoned' is the wordplay.
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'trained fruit tree' becomes 'espalier' (I've seen this before).
'abandoned' indicates named letters should be taken away.
'espalier' with 'i' taken away is 'espaler'.
'espaler' written backwards gives 'RELAPSE'.
'shows' acts as a link.
(Other definitions for relapse that I've seen before include "Return to sickness" , "Fall back into old ways" , "Backslide as in health matters" , "Become sick again" , "Regress, fall back" .)