A river with bats and sick old animal (9)
I believe the answer is:
armadillo
'animal' is the definition.
(I know that armadillo is a type of animal)
'a river with bats and sick old' is the wordplay.
'a river with' becomes 'ar' (I can't explain this - if you can you should believe this answer much more).
'bats' becomes 'mad' (bats can informally mean mad).
'and' says to put letters next to each other.
'sick' becomes 'ill' (synonyms).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'ar'+'mad'+'ill'+'o'='ARMADILLO'
Can you help me to learn more?
(Other definitions for armadillo that I've seen before include "Bony-plated mammal" , "Scaly burrowing animal" , "Animal covered with long plates" , "Insect-eating mammal" , "Long-snouted animal" .)