Upset dog, overweight, with the girl's sponsor at baptism (9)
I believe the answer is:
godfather
'sponsor at baptism' is the definition.
'godfather' can be an answer for 'sponsor' (I've seen this before). I am not certain of the 'at baptism' bit.
'upset dog overweight with the girl's' is the wordplay.
'upset' shows that the letters should be reversed in order.
'overweight' becomes 'fat' (similar in meaning).
'with' means one lot of letters go next to another.
'the girl's' becomes 'her' (belonging to a girl).
'dog' in reverse letter order is 'god'.
'god'+'fat'+'her'='GODFATHER'
Can you help me to learn more?
(Other definitions for godfather that I've seen before include "Baptismal sponsor" , "Religious relation, like Marlon Brando in movie" , "Mafia boss" , "leading exponent" , "gang boss" .)