A bad desire, that's understood (6)
I believe the answer is:
agreed
'understood' is the definition.
(thesaurus)
'a bad desire' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'desire' could be 'greed' (greed is a kind of desire) and 'greed' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for agreed that I've seen before include "came to the same conclusion" , "Shared an opinion" , "Accorded" , "Saw eye to eye" , "Jointly decided" .)