Working on ruin close to Heidelberg (5)
I believe the answer is:
doing
'working on' is the definition.
(I've seen this in another clue)
'ruin close to heidelberg' is the wordplay.
'ruin' becomes 'do in' (I've seen this in another clue).
'close to' suggests the final letters (the 'close' to the word).
The final letter of 'heidelberg' is 'g'.
'doin'+'g'='DOING'
(Other definitions for doing that I've seen before include "deed?" , "Beating (colloquial); performing" , "Execution" , "Engaged in -- active" , "thrashing" .)