Small lake sinks lower and lower (5)
I believe the answer is:
scowl
'lower' is the definition.
(scowling is a kind of lowering)
'small lake sinks lower' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lake' becomes 'L' (geographical abbreviation).
'sinks' means one lot of letters goes inside another (sink can mean to consume or drink).
'lower' becomes 'cow' (something which lows - makes cow sounds).
's'+'l'='sl'
'sl' enclosing 'cow' is 'SCOWL'.
'and' is the link.
(Other definitions for scowl that I've seen before include "Lower" , "Gloomy or threatening expression" , "Angry facial expression" , "Frown with displeasure" , "Look angry" .)