Look at companion in the lake (4)
I believe the answer is:
loch
'lake' is the definition.
(loch is a kind of lake)
'look at companion' is the wordplay.
'look' becomes 'lo' (archaic term meaning 'look!').
'at' means one lot of letters go next to another.
'companion' becomes 'ch' (Companion of Honour).
'lo'+'ch'='LOCH'
'in the' acts as a link.
(Other definitions for loch that I've seen before include "Scottish or Irish lake" , "Scottish word for lake" , "Ness or Lomond, for instance" , "A Scottish lake like Ness" , "Body of (Scottish?) water" .)