Beasts in a good lake descended on by soldiers (9)
I believe the answer is:
menagerie
'beasts' is the definition.
(a menagerie is a collection of animals)
'a good lake descended on by soldiers' is the wordplay.
'good' becomes 'g' (abbreviation).
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'descended on by' says to put letters next to each other (in a down clue, some letters descend onto others).
'soldiers' becomes 'men' (men can mean a group of soldiers).
'a'+'g'+'erie'='agerie'
'agerie' put after 'men' is 'MENAGERIE'.
'in' acts as a link.
(Other definitions for menagerie that I've seen before include "Animals go in here" , "Animal exhibition" , "strange collection of people" , "A lot of wild animals" , "collected animals" .)