After time, notice European youngster (7)
I believe the answer is:
tadpole
'youngster' is the definition.
(I've seen this before)
'after time notice european' is the wordplay.
'after' says to put letters next to each other.
'time' becomes 't' (abbreviation).
'notice' becomes 'ad' (both can mean an advertisement).
'european' becomes 'pole' (someone from Poland).
't'+'ad' is 'tad'.
'tad'+'pole'='TADPOLE'
(Other definitions for tadpole that I've seen before include "Baby frog" , "To plead for tiny frog" , "Potential frog" , "Old tape (anag.)" , "Young Kermit" .)