Deterioration of one falling out of tree upside down (7)
I believe the answer is:
relapse
'deterioration' is the definition.
(I've seen this before)
'one falling out of tree upside down' is the wordplay.
'one' becomes 'i' (Roman numeral).
'falling out of' indicates named letters should be taken away.
'tree' becomes 'espalier' (tree grown with a lattice).
'upside down' is a reversal indicator.
'espalier' with 'i' taken out is 'espaler'.
'espaler' reversed gives 'RELAPSE'.
'of' is the link.
(Other definitions for relapse that I've seen before include "Deteriorate after getting better" , "Become ill again" , "Fall back like leapers" , "Pleaser may slip to earlier state" , "Medical setback" .)