Composer’s hard day, struggling by noon (5)
I believe the answer is:
haydn
'composer's' is the definition.
(Haydn is an example)
'hard day struggling by noon' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'struggling' indicates an anagram.
'by' says to put letters next to each other.
'noon' becomes 'n'.
'day' anagrammed gives 'ayd'.
'h'+'ayd'+'n'='HAYDN'
(Other definitions for haydn that I've seen before include "Composer of over 100 symphonies" , "Austrian musician" , "Austrian composer of over one hundred symphonies" , "Creation composer" , "Josef --, prolific composer" .)