Chap about to hiss, as in self-assertive style (5)
I believe the answer is:
bossy
'chap about to hiss as in self-assertive style' is the definition.
I can't tell whether this defines the answer.
'chap about to' is the wordplay.
'chap' becomes 'Bo'.
'about to' becomes 'ssy' (I can't explain this - if you can you should believe this answer much more).
'bo'+'ssy'='BOSSY'
Can you help me to learn more?
(Other definitions for bossy that I've seen before include "As overbearing type" , "authoritarian" , "Domineering (colloq.)" , "Autocratic" , "Disagreeably domineering" .)