Feeble person taken aback by mathematician showing off (9)
I believe the answer is:
Here is my best explanation:
'showing off' is the definition.
'feeble person taken aback by mathematician' is the subsidiary indication.
'feeble person' becomes 'sop'.
'taken aback' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'mathematician' becomes 'turing' (Alan Turing).
'sop' backwards is 'pos'.
(Another definition for posturing that I've seen is "showing off".)