Line broken by a foreign body (5)
I believe the answer is:
alien
'foreign body' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'line broken by a' is the wordplay.
'broken' is an anagram indicator.
'by' means one lot of letters go next to another.
'line' with letters rearranged gives 'lien'.
'lien' put after 'a' is 'ALIEN'.
Can you help me to learn more?
(Other definitions for alien that I've seen before include "Stranger" , "Foreign, strange" , "immigrant" , "Clanger?" , "of foreigners" .)