Upended garden feature over time becomes liability (4)
I believe the answer is:
'liability' is the definition.
(debt is a kind of liability)
'upended garden feature over time' is the wordplay.
'upended' shows that the letters should be reversed in order (in a down clue, letters go up).
'garden feature' becomes 'bed' (as in a flower bed).
'over' means one lot of letters go next to another.
'time' becomes 't'.
'bed' back-to-front is 'deb'.
'becomes' acts as a link.
(Other definitions for debt that I've seen before include "Intertwine" , "state of obligation" , "What's due" , "Financial problem?" , "Liability, obligation" .)