Nancy's milk at home one's knocked back first (7)
I believe the answer is:
initial
'first' is the definition.
(similar in meaning)
'nancy's milk at home one's knocked back' is the wordplay.
'nancy's milk' becomes 'lait' (I can't explain this - if you can you should believe this answer much more).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'one' becomes 'i' (Roman numeral).
'knocked back' says the letters should be written backwards.
'lait'+'in'+'i'='laitini'
'laitini' in reverse letter order is 'INITIAL'.
Can you help me to learn more?
(Other definitions for initial that I've seen before include "Introductory" , "First letter of a name" , "Original; beginning" , "Occurring at the beginnng" , "Name's first letter" .)