Better witness finally recalling reg number right (8)
I believe the answer is:
stronger
'better' is the definition.
(stronger can mean of better quality or credentials)
'witness finally recalling reg number right' is the wordplay.
'finally' says to take the final letters.
'recalling' says the letters should be written in reverse.
'number' becomes 'no' (abbreviation for 'number').
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
The final letter of 'witness' is 's'.
'reg'+'no'+'rt'='regnort'
'regnort' backwards is 'tronger'.
's'+'tronger'='STRONGER'
(Other definitions for stronger that I've seen before include "Having greater power" , "Mightier, stouter" , "More potent" , "Less weak" , "More robust" .)