Agent sat out to vegetate (8)
I believe the answer is:
stagnate
'to vegetate' is the definition.
(thesaurus)
'agent sat out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'agent'+'sat'='agentsat'
'agentsat' anagrammed gives 'STAGNATE'.
(Other definitions for stagnate that I've seen before include "Fester" , "resemble pond water?" , "Lapse into inactivity" , "Become still and stale, pond or people (8)" , "Become stale or inactive" .)