The most common and most likely answer to this crossword clue is the 9 letter word LACTATION
I've seen this crossword clue in Financial Times
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The straight part
We know that "Mother’s release" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mother’s release = lactation.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LACTATION mean?
LACTATION noun- The secretion of milk from the mammary gland of a female mammal.
- The process of providing the milk to the young; breastfeeding.
- The period of time that a mother lactates to feed her young; lactation period.