Foreign lake that is covered by article (5)
I believe the answer is:
alien
'foreign' is the definition.
(synonyms)
'lake that is covered by article' is the wordplay.
'lake' becomes 'lien' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that is covered by' means one lot of letters go next to another (on top of in a down clue).
'article' becomes 'a' (the indefinite article in English).
'lien' put after 'a' is 'ALIEN'.
Can you help me to learn more?
(Other definitions for alien that I've seen before include "Martian, say" , "coming from overseas" , "Foreigner, outsider" , "movie!" , "Little green man, perhaps" .)