Someone retreating right ahead of artilleryman — not good (6)
I believe the answer is:
runner
'someone retreating' is the definition.
'right ahead of artilleryman not good' is the wordplay.
'right' becomes 'r' (common abbreviation).
'ahead of' says to put letters next to each other (I've seen this in other clues).
'artilleryman' becomes 'gunner' ('gunner' can be a synonym of 'artilleryman').
'not' suggests deleting specific letters.
'good' becomes 'g' (abbreviation).
'gunner' with 'g' taken out is 'unner'.
'r'+'unner'='RUNNER'
(Other definitions for runner that I've seen before include "Type of bean for an athlete?" , "Horse in a race; shoot" , "Harrier" , "Roadworthy vehicle" , "Percussion instrument" .)