Fish from river eaten by guest on rampage (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(sturgeon is a kind of fish)
'river eaten by guest on rampage' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'eaten by' is an insertion indicator.
'rampage' indicates anagramming the letters (I've seen 'rampaging' mean this).
'guest'+'on'='gueston'
'gueston' anagrammed gives 'stugeon'.
'r' put inside 'stugeon' is 'STURGEON'.
'from' acts as a link.
(Other definitions for sturgeon that I've seen before include "Fish yielding caviar and isinglass" , "So urgent to find caviar-bearing fish" , "Caviare-yielding fish" , "So urgent to find caviare-bearing fish" , "Fish from which caviar is obtained" .)