Bold porter regularly displaying extravagance (12)
I believe the answer is:
presumptuous
'bold' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for presumptuous that I've seen before include "Bold - forward" , "Impertinent" , "Assuming too much" , "Impertinently bold" .)