Old lady and thin Tony face off in marriage (9)
I believe the answer is:
matrimony
'in marriage' is the definition.
'matrimony' can be an answer for 'marriage' (synonyms). I am not certain of the 'in' bit.
'old lady and thin tony face' is the wordplay.
'old lady' becomes 'ma' (both can mean mother).
'and' means one lot of letters go next to another.
'thin' becomes 'trim' (similar in meaning).
'tony face' becomes 'ony' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ma'+'trim'+'ony'='MATRIMONY'
'off' acts as a link.
Can you help me to learn more?
(Other definitions for matrimony that I've seen before include "The state of wedlock" , "State of being married" , "wedding ceremony" , "Nuptials" .)