Poems most peculiar, but not every 3rd (4)
I believe the answer is:
odes
'poems' is the definition.
(ode is a kind of poem)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for odes that I've seen before include "poetry" , "Lines" , "Keats was fond of them" , "Lyrical poems" , "To a Skylark, To a Nightingale etc" .)