Cover of avant-garde album about river eels, oddly (8)
I believe the answer is:
umbrella
'cover' is the definition.
(I've seen this before)
'avant-garde album about river eels oddly' is the wordplay.
'avant-garde' indicates an anagram (a new or avant-garde version of the letters).
'about' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'eels' are 'el'.
'album' with letters rearranged gives 'umbla'.
'r'+'el'='rel'
'umbla' going around 'rel' is 'UMBRELLA'.
'of' is the link.
(Other definitions for umbrella that I've seen before include "Overall agency" , "Type of pine tree" , "What goes up when rain comes down?" , "Protecting cover" , "To get protection" .)