Doofus gets jail after beheading attack (6)
I believe the answer is:
assail
'attack' is the definition.
(synonyms)
'doofus gets jail after beheading' is the wordplay.
'doofus' becomes 'ass' (I've seen this in another clue).
'gets' means one lot of letters go next to another.
'after beheading' means to remove the first letter.
'jail' with its initial letter removed is 'ail'.
'ass'+'ail'='ASSAIL'
(Other definitions for assail that I've seen before include "Molest or harass" , "storm" , "Assault, attack" , "Sudden attack" , "Launch an attack" .)