European river crossed by nerdy enthusiast (5)
I believe the answer is:
greek
'european' is the definition.
(I know that Greek is a type of european)
'river crossed by nerdy enthusiast' is the wordplay.
I cannot really understand how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is found in the answer.
'enthusiast' could be 'geek' (I have seen 'Obsessive enthusiast ' mean 'geek' so perhaps 'enthusiast' could also mean 'geek') and 'geek' is present in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for greek that I've seen before include "Nationality" , "One from Salonika" , "Athenian?" , "Foreigner" , "Supposedly unknown language?" .)