School uniform summer shirt range (5)
I believe the answer is:
gamut
'range' is the definition.
(gamut can mean a range or variety)
'school uniform summer shirt' is the wordplay.
'school' becomes 'gam' (both can refer to a group of aquatic mammals).
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'summer shirt' becomes 't' (as in a T-shirt. I am not sure about the 'summer' bit.).
'gam'+'u'+'t'='GAMUT'
Can you help me to learn more?
(Other definitions for gamut that I've seen before include "Ambit" , "Entire scope" , "Complete range or scale" , "Entire scale" , "The whole range of, say emotions" .)