In conclusion, boss rules roost ultimately (6)
I believe the answer is:
result
'in conclusion' is the definition.
'result' can be an answer for 'conclusion' (result is a kind of conclusion). I am unsure of the 'in' bit.
'boss rules roost ultimately' is the wordplay.
I cannot quite see how this works, but
an anagram of 'rules' is 'resul' which is present in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for result that I've seen before include "Effect of lustre" , "Consequence or effect" , "Verdict" , "Final outcome" , "Rustle (anag.)" .)