Evasive firm, untrustworthy ultimately (3)
I believe the answer is:
coy
'evasive' is the definition.
'firm untrustworthy ultimately' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'ultimately' says to take the final letters.
The last letter of 'untrustworthy' is 'y'.
'co'+'y'='COY'
(Other definitions for coy that I've seen before include "Pretending to be shy or bashful" , "reluctant to divulge information" , "Reserved" , "Pretending shyness" , "Pretending modesty" .)