One who steals mother's good food (6)
I believe the answer is:
magpie
'one who steals' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'mother's good food' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'good' becomes 'g' (abbreviation).
'food' becomes 'pie' (pie is a kind of food).
'ma'+'g'+'pie'='MAGPIE'
Can you help me to learn more?
(Other definitions for magpie that I've seen before include "winger" , "1 [UNLUCKY FOR SOME]" , "Ominous bird if seen alone" , "Black-and-white crow" , "said to bring sorrow" .)