Taxonomist in mid-life gets posher housing (10)
I believe the answer is:
classifier
'taxonomist' is the definition.
(someone who classifies within a taxonomy)
'mid-life gets posher housing' is the wordplay.
'mid' means to look at the middle letters.
'life' becomes 'bio' (life can mean a biography).
'gets' becomes 'f' (I can't justify this - if you can you should give a lot more credence to this answer).
'posher' becomes 'classier' ('classy' can be a synonym of 'posh').
'housing' means one lot of letters goes inside another.
The middle of 'bio' is 'i'.
'i'+'f'='if'
'if' placed inside 'classier' is 'CLASSIFIER'.
'in' is the link.
(Another definition for classifier that I've seen is " Someone ordering".)