Dilapidated ruin, one abandoned on an upland (3-4)
I believe the answer is:
run-down
'dilapidated' is the definition.
(I've seen this before)
'ruin one abandoned on an upland' is the wordplay.
'one' becomes 'i' (Roman numeral).
'abandoned' indicates named letters should be taken away.
'on' is a charade indicator (letters next to each other).
'an upland' becomes 'down' (down is a kind of upland).
'ruin' with 'i' taken out is 'run'.
'run'+'down'='RUN-DOWN'
(Other definitions for run-down that I've seen before include "Exhausted" , "In a neglected state" , "Shabby" , "Worn-out" , "Dilapidated" .)