Composer has caught cold on hot, hot day (8)
I believe the answer is:
scorcher
'hot day' is the definition.
(I've seen this before)
'composer has caught cold on hot' is the wordplay.
'composer' becomes 'scorer' (I can't explain this - if you can you should give a lot more credence to this answer).
'has caught' means one lot of letters goes inside another.
'cold' becomes 'c' (eg on taps).
'on' is a charade indicator (letters next to each other).
'hot' becomes 'h' (abbreviation eg on taps).
'c'+'h'='ch'
'scorer' placed around 'ch' is 'SCORCHER'.
Can you help me to learn more?
(Other definitions for scorcher that I've seen before include "burner" , "Day of very hot and sunny weather" , "something remarkable" , "Informally, a really hot day - it burns!" , "heat wave" .)