Lower farm animal into small lake (5)
I believe the answer is:
scowl
'lower' is the definition.
(scowling is a kind of lowering)
'farm animal into small lake' is the wordplay.
'farm animal' becomes 'cow' (type of farm animal).
'into' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lake' becomes 'L' (geographical abbreviation).
's'+'l'='sl'
'cow' going within 'sl' is 'SCOWL'.
(Other definitions for scowl that I've seen before include "Look threatening" , "Glower; frown" , "Bad-tempered expression" , "Glare" , "Grimace" .)