European intelligence agency goes after information source (7)
I believe the answer is:
genesis
'source' is the definition.
(I've seen this in another clue)
'european intelligence agency goes after information' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'intelligence agency' becomes 'SIS' (Secret Intelligence Service, official name of MI6).
'goes after' means one lot of letters go next to another.
'information' becomes 'gen' (gen can informally mean information).
'e'+'sis'='esis'
'esis' put after 'gen' is 'GENESIS'.
(Other definitions for genesis that I've seen before include "Creation" , "rock band" , "First book of the Old Testament" , "Beginning the holy book" , "the start of something" .)