River creature is warmer, but not at first (5)
I believe the answer is:
otter
'river creature' is the definition.
(I've seen this before)
'warmer but not at first' is the wordplay.
'warmer' becomes 'hotter' ('hot' can be a synonym of 'warm').
'but not at first' means to remove the first letter.
'hotter' with its first letter removed is 'OTTER'.
'is' is the link.
(Other definitions for otter that I've seen before include "Fish-eating water animal with brown fur and flat tail" , "Fur possibly" , "eg Tarka" , "Aquatic creature" , "Web-footed mammal" .)