Ornamental tree, one cut back, is to deteriorate (7)
I believe the answer is:
relapse
'is to deteriorate' is the definition.
(as in a relapse of a medical condition)
'ornamental tree one cut back' is the wordplay.
'ornamental tree' becomes 'espalier' (type of tree).
'one' becomes 'i' (Roman numeral).
'cut' suggests deleting specific letters.
'back' is a reversal indicator.
'espalier' with 'i' taken away is 'espaler'.
'espaler' in reverse letter order is 'RELAPSE'.
(Other definitions for relapse that I've seen before include "Repeals (anag.)" , "Fall back into illness" , "Pleaser may slip to earlier state" , "Fall back into old ways" , "Decline" .)