Hardline barrister initially involved in interminable case (9)
I believe the answer is:
unbending
'hardline' is the definition.
(both can mean extreme or inflexible)
'barrister initially involved in interminable case' is the wordplay.
'initially' indicates taking the first letters.
'involved in' indicates putting letters inside.
'interminable case' becomes 'unending' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letter of 'barrister' is 'b'.
'b' placed into 'unending' is 'UNBENDING'.
(Another definition for unbending that I've seen is " Rigid".)