In fact, two hands should be replaced by another professional (7)
I believe the answer is:
actuary
'professional' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for actuary that I've seen before include "Insurance agent" , "Person using statistics to calculate insurance premiums" , "Someone who analyses statistics to calculate insurance risks" , "Professional calculator of insurance statistics" , "Insurance adviser" .)