European enthusiast up for song (4)
I believe the answer is:
tune
'song' is the definition.
(I've seen this in another clue)
'european enthusiast up' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'enthusiast' becomes 'nut' (nut can mean an enthusiast or fan).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'e'+'nut'='enut'
'enut' written backwards gives 'TUNE'.
'for' acts as a link.
(Other definitions for tune that I've seen before include "Improve quality" , "strain" , "Adjust for better performance" , "Song" , "Adjust a radio receiver to a frequency" .)