Animal by a lake ate salt (7)
I believe the answer is:
oxalate
'salt' is the definition.
(I know that oxalate is a type of salt)
'animal by a lake ate' is the wordplay.
'animal' becomes 'ox' (castrated bull).
'by' says to put letters next to each other.
'lake' becomes 'L' (geographical abbreviation).
'ox'+'a'+'l'+'ate'='OXALATE'
(Other definitions for oxalate that I've seen before include "Acid salt; a lax toe (anag.)" , "that's salty!" .)