Friend arranged to frame artists 'A Piece of the Spectrum'
I believe the answer is:
infrared
'spectrum' is the definition.
(infrared spectrum is a kind of spectrum)
'friend arranged to frame artists' is the wordplay.
'arranged' is an anagram indicator.
'to frame' means one lot of letters goes inside another.
'artists' becomes 'RA' (Royal Academy of the Arts).
'friend' with letters rearranged gives 'infred'.
'infred' enclosing 'ra' is 'INFRARED'.
'a piece of the' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for infrared that I've seen before include "(Of radiation) with a longer wavelength than light" , "sort of photography" , "With a frequency between the red end of the visible spectrum and microwaves" , "Fan drier (anag) - type of radiation" , "Radiation from heated objects" .)