Witness turned up to admit politician bears stresses (8)
I believe the answer is:
emphases
'stresses' is the definition.
('emphasis' can be a synonym of 'stress')
'witness turned up to admit politician bears' is the wordplay.
'witness' becomes 'see' (I've seen this before).
'turned up' says the letters should be written in reverse.
'to admit' indicates putting letters inside.
'politician' becomes 'mp'.
'bears' becomes 'has' ('have' can be a synonym of 'bear'**).
'see' back-to-front is 'ees'.
'mp'+'has'='mphas'
'ees' placed around 'mphas' is 'EMPHASES'.
(Another definition for emphases that I've seen is " Stresses".)