Play guitar, say, entertaining Charlie after gym — colourful display? (8)
I believe the answer is:
spectrum
'colourful display?' is the definition.
(spectrum of colours)
'play guitar say entertaining charlie after gym' is the wordplay.
'play guitar say' becomes 'strum' (strum can mean to play a guitar).
'entertaining' means one lot of letters goes inside another.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'after' means one lot of letters go next to another.
'gym' becomes 'pe' (short for Physical Education).
'c' put after 'pe' is 'pec'.
'strum' enclosing 'pec' is 'SPECTRUM'.
(Other definitions for spectrum that I've seen before include "Bands of colour in crumpets" , "Range of colours, opinions etc" , "Broad range of related items" , "Complete colour range" , "range of tones" .)