Old poem found under page eleven or thirteen? (5)
I believe the answer is:
prime
'eleven or thirteen?' is the definition.
I can't judge whether this defines the answer.
'old poem found under page' is the wordplay.
'old poem' becomes 'rime' (archaic spelling of 'rhyme').
'found under' is a charade indicator (letters next to each other).
'page' becomes 'p'.
'rime' put after 'p' is 'PRIME'.
Can you help me to learn more?
(Other definitions for prime that I've seen before include "Fundamental - of highest quality" , "Foremost" , "Number with no factor but itself and 1" , "Main; make ready" , "golden age?" .)