Lower a boat on the lake (5)
I believe the answer is:
scowl
'lower' is the definition.
(scowling is a kind of lowering)
'a boat on the lake' is the wordplay.
'a boat' becomes 'scow' (scow is a kind of boat).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'the lake' becomes 'L' (geographical abbreviation).
'scow'+'l'='SCOWL'
(Other definitions for scowl that I've seen before include "Angry frown" , "Grimace" , "Glare" , "Glower; frown" , "Bad-tempered expression" .)