Everybody in a large lake (3)
I believe the answer is:
all
'everybody' is the definition.
(thesaurus)
'in a large lake' is the wordplay.
'in' indicates putting letters inside.
'large' becomes 'l'.
'lake' becomes 'l'.
'a'+'l'='al'
'al' placed around 'l' is 'ALL'.
(Other definitions for all that I've seen before include "The whole quantity" , "The lot, everyone" , "Wholly" , "Passage in a formal garden" , "and 16 Across and 23 Across: Even so" .)