European tax company backed group of eight (6)
I believe the answer is:
octave
'group of eight' is the definition.
(I've seen this before)
'european tax company backed' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'tax' becomes 'vat' (British sales tax).
'company' becomes 'co' (abbreviation).
'backed' is a reversal indicator.
'e'+'vat'+'co'='evatco'
'evatco' backwards is 'OCTAVE'.
(Other definitions for octave that I've seen before include "Note an eighth above or below another" , "Group of eight lines" , "Span of eight notes" , "Series of eight notes in music" , "Musical term - set of eight" .)