Collected idiot found in a body of water (7)
I believe the answer is:
amassed
'collected' is the definition.
('amass' can be a synonym of 'collect')
'idiot found in a body of water' is the wordplay.
'idiot' becomes 'ass' (I've seen this before).
'found in' indicates putting letters inside.
'body of water' becomes 'med' (short for Mediterranean).
'a'+'med'='amed'
'ass' going within 'amed' is 'AMASSED'.
(Other definitions for amassed that I've seen before include "Accumulated, piled up" , "Got up" , "Accumulated a great quantity" , "Put together" , "Gathered together" .)