Colourful band finally uses guitarist’s aid left out (8)
I believe the answer is:
spectrum
'colourful band' is the definition.
'uses guitarist's aid left out' is the wordplay.
'uses' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'guitarist's aid' becomes 'plectrum' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'out' indicates named letters should be taken away.
'plectrum' with 'l' taken out is 'pectrum'.
's'+'pectrum'='SPECTRUM'
'finally' acts as a link.
This may not be correct. It may be part of another bit of the clue.
(Other definitions for spectrum that I've seen before include "An entire range of related qualities of crumpets" , "Complete colour range" , "'Broad range, scope (8)'" , "Crumpets can provide the full array of colours" , "Light split by prism" .)