Source of milk I'm beginning to exploit between lunch and dinner (7)
I believe the answer is:
teatime
'between lunch and dinner' is the definition.
(time for tea, meal between lunch and dinner)
'source of milk i'm beginning to exploit' is the wordplay.
'source of milk' becomes 'teat' (body part which dispenses milk).
'beginning to' indicates taking the first letters.
The initial letter of 'exploit' is 'e'.
'teat'+'im'+'e'='TEATIME'
(Other definitions for teatime that I've seen before include "Eat item (anag.)" , "Eat mite (anag.)" , "Four o'clock?" , "When cake might be had" , "Afternoon meal break" .)