Mammal, a second seen in small lake (6)
I believe the answer is:
weasel
'mammal' is the definition.
(weasel is a kind of mammal)
'a second seen in small lake' is the wordplay.
'second' becomes 's' (common abbreviation).
'seen in' means one lot of letters goes inside another.
'small' becomes 'wee' ('wee' can be similar in meaning to 'small'**).
'lake' becomes 'l'.
'a'+'s'='as'
'as' going within 'wee' is 'wease'.
'wease'+'l'='WEASEL'
(Other definitions for weasel that I've seen before include "Old jingle 'Pop goes the . . . . . .'" , "Mammal related to stoat" , "Carnivorous mammal; treacherous person" , "Small fierce animal with slender body and reddish fur" , "Rat" .)