Boss determined to be wilful (10)
I believe the answer is:
headstrong
'wilful' is the definition.
('headstrong' can be a synonym of 'wilful')
'boss determined' is the wordplay.
'boss' becomes 'head' ('head' can be a synonym of 'boss').
'determined' becomes 'strong' ('strong' can be similar in meaning to 'determined').
'head'+'strong'='HEADSTRONG'
'to be' acts as a link.
(Other definitions for headstrong that I've seen before include "obstinately self-willed" , "Obstinately rash" , "Obstinate" , "Wilful, determined" , "Wilfully determined" .)