Milk's left out back to remove fat (4)
I believe the answer is:
skim
'remove fat' is the definition.
'skim' can be an answer for 'remove' (skimming is a kind of removing). I am unsure of the 'fat' bit.
'milk's left out back' is the wordplay.
'left' becomes 'l' (common abbreviation).
'out' is a deletion indicator.
'back' shows that the letters should be reversed in order.
'milks' with 'l' removed is 'miks'.
'miks' back-to-front is 'SKIM'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for skim that I've seen before include "Remove surface layer" , "Remove top layer; read bits of" , "Bounce (a stone) over water" , "Remove floating matter from surface" , "Glance over" .)