Going across river involves guts (8)
I believe the answer is:
entrails
'guts' is the definition.
(thesaurus)
'going across river involves' is the wordplay.
'going across' means one lot of letters goes inside another.
'river' becomes 'r'.
'involves' becomes 'entails' (I've seen this before).
'r' put inside 'entails' is 'ENTRAILS'.
(Other definitions for entrails that I've seen before include "Internal organs of animals" , "Intestines of animals" , "Internal parts of animals" , "Innards, viscera" , "Bowels or intestines" .)