Ideal metre and poem length (5)
I believe the answer is:
model
'ideal' is the definition.
(I know that model is a type of ideal)
'metre and poem length' is the wordplay.
'metre' becomes 'm' (synonyms).
'and' says to put letters next to each other.
'poem' becomes 'ode' (ode is a type of poem).
'length' becomes 'l' (abbreviation in geometry).
'm'+'ode'+'l'='MODEL'
(Other definitions for model that I've seen before include "Scale replica" , "Small-scale version" , "Replica (on the catwalk?)" , "Possibly the last" , "dummy" .)