Glower when farm animal gets into small lake (5)
I believe the answer is:
scowl
'glower' is the definition.
(I know that scowl has the is a more specific form of the action glower)
'farm animal gets into small lake' is the wordplay.
'farm animal' becomes 'cow' (type of farm animal).
'gets into' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lake' becomes 'L' (geographical abbreviation).
's'+'l'='sl'
'cow' going within 'sl' is 'SCOWL'.
'when' acts as a link.
(Other definitions for scowl that I've seen before include "Look threatening" , "Bad-tempered expression" , "Glare" , "Lower" , "Grimace" .)