Fancying one is boss is a consuming passion (9)
I believe the answer is:
obsession
'consuming passion' is the definition.
'obsession' can be an answer for 'passion' (thesaurus). I am not sure about the 'consuming' bit.
'fancying one is boss' is the wordplay.
'fancying' indicates anagramming the letters (I've seen 'fancied' mean this (a fancy or special version of the letters)).
'one'+'is'+'boss'='oneisboss'
'oneisboss' anagrammed gives 'OBSESSION'.
'is a' is the link.
Can you help me to learn more?
(Other definitions for obsession that I've seen before include "Enthusiasm" , "All-absorbing idea" , "Compulsion, fixation" , "Mania" , "Extreme interest" .)