Girls excited by European youngster (6)
I believe the answer is:
grilse
'youngster' is the definition.
Both the answer and definition are singular nouns.
Maybe there's a link between them I don't understand?
'girls excited by european' is the wordplay.
'excited' is an anagram indicator.
'by' says to put letters next to each other.
'european' becomes 'E' (abbreviation e.g. EU).
'girls' is an anagram of 'grils'.
'grils'+'e'='GRILSE'
Can you help me to learn more?
(Other definitions for grilse that I've seen before include "Young salmon (6)" , "A one-year old salmon" , "Young swimmer" , "Salmon on first return to fresh water" , "Young salmon returning from sea to spawn" .)