Classic perhaps, ends in wondrous line, corker penned by old poet (5,4)

'classic perhaps' is the definition.
(UK horseracing has five 'Classic' races)

'ends in wondrous line corker penned by old poet' is the wordplay.
'ends in' says to take the final letters (I've seen 'end in' mean this).
'penned by' means one lot of letters goes inside another.
'old poet' becomes 'horace'.
The last letters of 'wondrous line corker' are 'ser'.
'ser' put into 'horace' is 'HORSE-RACE'.