Principles not constant in lyric poem (3)
I believe the answer is:
ode
'lyric poem' is the definition.
(ode is a kind of lyric poem)
'principles not constant' is the wordplay.
'principles' becomes 'code' (a moral code).
'not' indicates named letters should be taken away.
'constant' becomes 'c' (mathematical abbreviation).
'code' with 'c' removed is 'ODE'.
'in' acts as a link.
(Other definitions for ode that I've seen before include "Verse form" , "A poem celebrating person or event" , "poetry" , "Literary work" , "Poetic composition" .)