Pi chaps unexpectedly like some of Horace’s odes (7)
I believe the answer is:
sapphic
'like some of horace's odes' is the definition.
(Horace sometimes wrote Sapphic verse)
'pi chaps unexpectedly' is the wordplay.
'unexpectedly' indicates an anagram.
'pi'+'chaps'='pichaps'
'pichaps' is an anagram of 'SAPPHIC'.
(Other definitions for sapphic that I've seen before include "Poetic gem" , "related to lyric poet" , "emale poetry" , "Type of poetry" , "Relating to a Greek female poet" .)