A shallow lake across the sea, perhaps? (6)
I believe the answer is:
abroad
'across the sea perhaps?' is the definition.
(I've seen this in another clue)
'a shallow lake' is the wordplay.
'shallow lake' becomes 'broad' (e.g. in the Norfolk broads).
'a'+'broad'='ABROAD'
(Other definitions for abroad that I've seen before include "in Paris, perhaps" , "Europe, for example" , "Stare lecherously at (someone)" , "At large - overseas" , "Overseas; (of rumour) widely current" .)