A little water rat I classified as wandering (7)
I believe the answer is:
erratic
'wandering' is the definition.
(I know that wandering can be written as erratic)
'a little water rat i classified' is the wordplay.
'a little' indicates the answer is hidden within the clue (a little bit of the clue).
'ERRATIC' is hidden within 'water rat i classified'.
'as' acts as a link.
(Other definitions for erratic that I've seen before include "Inconstant" , "Eccentric, aberrant" , "Wandering" , "Inconsistent, shifting" , "Volatile" .)