Old establishment regiment gone off to get a hormone (9)
I believe the answer is:
oestrogen
'hormone' is the definition.
(I know that oestrogen is a type of sex hormone)
'old establishment regiment gone off to get a' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found in the answer.
an anagram of 'gone' is 'ogen' which is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for oestrogen that I've seen is " Hormone".)