Wealthy US proposer involved (10)
I believe the answer is:
prosperous
'wealthy' is the definition.
(thesaurus)
'us proposer involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'us'+'proposer'='usproposer'
'usproposer' anagrammed gives 'PROSPEROUS'.
(Other definitions for prosperous that I've seen before include "Wealthy" , "thriving" , "Rich" , "Well-to-do" , "Affluent" .)