Former partner with company, one brought in to evaluate and criticise (9)
I believe the answer is:
excoriate
'criticise' is the definition.
(to excoriate is to criticise severely)
'former partner with company one brought in to evaluate' is the wordplay.
I cannot really understand how this works, but
'former' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
'partner' could be 'e' and 'e' is located in the answer.
'company' could be 'co' (abbreviation) and 'co' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
The remaining letters 'rat' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'and' acts as a link.
(Other definitions for excoriate that I've seen before include "roast" , "Flay" .)