Firm has corrupt data about staff (7)
I believe the answer is:
adamant
'firm' is the definition.
(adamant can mean firm or determined)
'corrupt data about staff' is the wordplay.
'corrupt' is an anagram indicator (a corrupted spelling of the word).
'about' means one lot of letters goes inside another.
'staff' becomes 'man' (as in staffing or manning a particular place).
'data' is an anagram of 'adat'.
'adat' going around 'man' is 'ADAMANT'.
'has' acts as a link.
(Other definitions for adamant that I've seen before include "Resolute, refusing to yield" , "Refusing to change one's mind" , "Unshakable (in an opinion)" , "Unyielding to requests, stubborn" , "Data man can be quite unyielding" .)