Mammal, timid and occasionally nasty, circling river (7)
I believe the answer is:
meerkat
'mammal' is the definition.
(meerkat is a kind of mammal)
'timid and occasionally nasty circling river' is the wordplay.
'timid' becomes 'meek' (I've seen this before).
'and' says to put letters next to each other.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'circling' means one lot of letters goes inside another (circling can mean going around or containing).
'river' becomes 'r' (abbreviation).
The alternating letters of 'nasty' are 'at'.
'meek'+'at'='meekat'
'meekat' going around 'r' is 'MEERKAT'.
(Other definitions for meerkat that I've seen before include "Small South American cat-like animal, has four toes" , "Rake met (anag) - type of mongoose" , "Mongoose-like mongoose" , "Small mongoose of southern Africa" , "S. African animal" .)