Obvious mistake -- a baby could be one (6)
I believe the answer is:
howler
I believe this clue is a double definition.
'obvious mistake' is the first definition.
'a baby could be one' is the second definition.
(a baby might howl)
(Other definitions for howler that I've seen before include "Extremely funny joke or glaring blunder" , "Serious blunder" , "Schoolboy error - wolf?" , "A monkey; a bad mistake" , "A foolish mistake, sounds noisy" .)