A youngster's endless poem is in fashion (1,2,4)
I believe the answer is:
a la mode
'in fashion' is the definition.
(I've seen this before)
'a youngster's endless poem' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'poem' could be 'ode' (ode is a type of poem) and 'ode' is found in the answer.
The remaining letters 'lam' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for a la mode that I've seen before include "Fashionable in France" , "In the current fashion" , "In fashion - served with ice cream" , "Stylish" , "stewed with veg?" .)