Range of mushrooms sent back with cap trimmed (8)
I believe the answer is:
'range' is the definition.
(a spectrum is a range or variety)
I cannot understand how the remainder of the clue works.
(Other definitions for spectrum that I've seen before include "Complete colour range" , "Band of refracted colours" , "An entire range of related qualities of crumpets" , "Crumpets can provide the full array of colours" , "Bands of colour in crumpets" .)