Lay into a girl coming back about one! (6)
I believe the answer is:
assail
'lay into' is the definition.
(I've seen this in another clue)
'a girl coming back about one' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'coming back' says the letters should be written backwards.
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'lass' in reverse letter order is 'ssal'.
'ssal' going around 'i' is 'ssail'.
'a'+'ssail'='ASSAIL'
(Other definitions for assail that I've seen before include "storm" , "Criticize" , "Attack, lash out" , "Launch an attack" , "Molest or harass" .)