Hooligan beheaded with Old English instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(oboe is a kind of instrument)
'hooligan beheaded with old english' is the wordplay.
'hooligan' becomes 'yob' (synonyms).
'beheaded' means to remove the first letter (the head of the word is removed).
'with' means one lot of letters go next to another.
'old english' becomes 'oe'.
'yob' with its first letter removed is 'ob'.
'ob'+'oe'='OBOE'
(Other definitions for oboe that I've seen before include "Producer of notes" , "blow it!" , "Walk with exaggerated movements" , "Double-reed wind instrument" , "source of notes" .)