Fish missing an hour, going astray (6)
I believe the answer is:
erring
'going astray' is the definition.
(err can mean to go the wrong way)
'fish missing an hour' is the wordplay.
'fish' becomes 'herring' (herring is a type of fish).
'missing an' suggests deleting specific letters.
'hour' becomes 'h' (abbreviation).
'herring' with 'h' taken out is 'ERRING'.
(Other definitions for erring that I've seen before include "Being incorrect" , "Making 7 down" , "Going wrong" , "taking wrong direction" , "Making a mistake" .)