An excessive desire, that's understood (6)
I believe the answer is:
agreed
'understood' is the definition.
(thesaurus)
'an excessive desire' is the wordplay.
I cannot quite see how this works, but
'an' could be 'a' and 'a' is found in the answer.
'desire' could be 'greed' (greed is a kind of desire) and 'greed' is found within the answer.
This accounts for all the letters.
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 "Let's do it!" , "Tallied - reconciled" , "ratified" , "Shared an opinion" , "acceptable" .)