Mammal observed peak over lake, looking up (5,3)
I believe the answer is:
brown rat
'mammal' is the definition.
(brown rat is a kind of mammal)
'observed peak over lake looking up' is the wordplay.
'observed peak over' becomes 'brow' (I am not sure about this - if you are sure you should believe this answer much more).
'lake' becomes 'tarn' (tarn is a kind of lake).
'looking up' says the letters should be written in reverse (in a down clue, letters go up).
'tarn' reversed gives 'nrat'.
'brow'+'nrat'='BROWN RAT'
Can you help me to learn more?
(Another definition for brown rat that I've seen is " Common rodent pest".)