US stock market regulator head promoted under European principles (8)
I believe the answer is:
essences
'principles' is the definition.
(principles are essential values or qualities)
'us stock market regulator head promoted under european' is the wordplay.
'us stock market regulator' becomes 'SEC' (Securities and Exchange Commission).
'head' becomes 'ness' (ness is a headland).
'promoted' says the letters should be written in reverse (going up, in a down clue).
'under' says to put letters next to each other.
'european' becomes 'E' (abbreviation e.g. EU).
'sec'+'ness'='secness'
'secness' back-to-front is 'ssences'.
'ssences' put after 'e' is 'ESSENCES'.
(Other definitions for essences that I've seen before include "Plant extracts" , "Perfumes and concentrated extracts" , "Perfumes of concentrates" , "Extracts or fundamental natures" , "Concentrated extracts of plants, for perfume say" .)