Headless fish going astray (6)
I believe the answer is:
erring
'going astray' is the definition.
(err can mean to go the wrong way)
'headless fish' is the wordplay.
'headless' means to remove the first letter.
'fish' becomes 'herring' (herring is a type of fish).
'herring' with its first letter taken away is 'ERRING'.
(Other definitions for erring that I've seen before include "taking wrong direction" , "Making 7 down" , "Going wrong" , "Being incorrect" , "Sinful" .)