Firm's staff probing corrupt data (7)
I believe the answer is:
adamant
'firm's' is the definition.
(adamant can mean firm or determined)
'staff probing corrupt data' is the wordplay.
'staff' becomes 'man' (as in staffing or manning a particular place).
'probing' is an insertion indicator (probe can mean to poke or prod into).
'corrupt' is an anagram indicator (a corrupted form).
'data' is an anagram of 'adat'.
'man' put within 'adat' is 'ADAMANT'.
(Other definitions for adamant that I've seen before include "popstar" , "Unyielding to requests, stubborn" , "Hard stone" , "Legendary rock" , "Positive or intransigent" .)