Aquatic animal of type that enjoys river, first of all (5)
I believe the answer is:
otter
'aquatic animal' is the definition.
(I've seen this before)
'of type that enjoys river first of all' is the wordplay.
'of' becomes 'o'.
'first of all' indicates taking the first letters.
The initial letters of 'type that enjoys river' is 'tter'.
'o'+'tter'='OTTER'
(Other definitions for otter that I've seen before include "Fish eater of the weasel family" , "Mammal, 'Tarka'" , "fish-eater displayed" , "Riverine mammal" , "furry animal" .)