Instrument used by a bonehead to stir glue! (5)
I believe the answer is:
bugle
'instrument' is the definition.
(bugle is a kind of instrument)
'by a bonehead to stir glue' is the wordplay.
'by' means one lot of letters go next to another.
'a bonehead' becomes 'b' (head letter of 'bone').
'to stir' is an anagram indicator.
'glue' with letters rearranged gives 'ugle'.
'b'+'ugle' is 'BUGLE'.
'used' is the link.
(Other definitions for bugle that I've seen before include "Brass instrument without valves" , "Bit of brass" , "Horn" , "Small trumpet; plant" , "Creeping plant with blue flowers" .)