Profession involving writer turned soldier (9)
I believe the answer is:
carbineer
'soldier' is the definition.
(I know that Carbineer is a type of soldier)
'profession involving writer turned' is the wordplay.
'profession' becomes 'career' (someone's profession is their career or occupation).
'involving' is an insertion indicator.
'writer' becomes 'nib' (something which writes).
'turned' says the letters should be written backwards.
'nib' back-to-front is 'bin'.
'career' placed around 'bin' is 'CARBINEER'.
(Another definition for carbineer that I've seen is " Soldier".)