Milk supplier's entertaining second commercial sent over computer file (4,3)
I believe the answer is:
data set
'computer file' is the definition.
(set of data on a computer)
'milk supplier's entertaining second commercial sent over' is the wordplay.
'milk supplier's' becomes 'teat' (I can't explain this - if you can you should believe this answer much more).
'entertaining' is an insertion indicator (inviting in, as for dinner).
'second' becomes 's' (common abbreviation).
'commercial' becomes 'ad' (abbreviation for advertisement).
'sent over' says the letters should be written in reverse.
'teat' placed around 's' is 'tesat'.
'tesat'+'ad'='tesatad'
'tesatad' reversed gives 'DATA SET'.
Can you help me to learn more?
(Other definitions for data set that I've seen before include "collected info" , "Collection of information (in computing)" , "file" .)