Haydn's first symphony almost epic (6)
I believe the answer is:
heroic
'epic' is the definition.
(synonyms)
'haydn's first symphony almost' is the wordplay.
'haydn's first' becomes 'h' (1st letter of 'haydn').
'symphony' becomes 'eroica' (Beethoven's third symphony).
'almost' means to remove the last letter (almost the whole word is kept).
'eroica' with its last letter taken off is 'eroic'.
'h'+'eroic'='HEROIC'
(Other definitions for heroic that I've seen before include "Superhuman (efforts)" , "Surpassing the ordinary, in courage say" , "larger than life" , "Dramatic" , "Showing extreme courage" .)