Man loves probing margins of field for mice, say (3,4)
I believe the answer is:
cat food
'for mice say' is the definition.
(a cat might eat mice)
'man loves probing margins of field' is the wordplay.
'man' becomes 'cat' (American slang for man, esp in Jazz).
'loves' becomes 'oo' (love means 0 in tennis).
'probing' means one lot of letters goes inside another (probe can mean to make a hole in).
'margins of' says to hollow out the word (remove centre letters) (only the letters on the margins of the word).
'field' with its middle taken out is 'fd'.
'oo' placed within 'fd' is 'food'.
'cat'+'food'='CAT FOOD'
(Another definition for cat food that I've seen is " Tom needs this".)