Not having been attended to, is ruined (6)
I believe the answer is:
undone
'is ruined' is the definition.
(synonyms)
'not having been' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'having' says to put letters next to each other.
'been' becomes 'done' (I can't justify this - if you can you should give a lot more credence to this answer).
'un'+'done'='UNDONE'
'attended to' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for undone that I've seen before include "Not finished; untied" , "Brought to ruin or destruction - or not buttoned" , "Not finished; ruined" , "Annulled; untied" , "Not bound" .)