Fast-moving agent takes in European editor (6)
I believe the answer is:
speedy
'fast-moving' is the definition.
(I've seen this before)
'agent takes in european editor' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'takes in' indicates putting letters inside.
'european' becomes 'e' (abbreviation e.g. EU).
'editor' becomes 'ed'.
'e'+'ed'='eed'
'spy' going around 'eed' is 'SPEEDY'.
(Other definitions for speedy that I've seen before include "'Fast, without delay (6)'" , "Quick" , "Swift" , "Fast-moving" , "Rapid, fast" .)