Firm about middle (4)
I believe the answer is:
core
'middle' is the definition.
(thesaurus)
'firm about' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'about' becomes 're' (regarding**).
'co'+'re'='CORE'
(Other definitions for core that I've seen before include "Central part of the Earth" , "essential element" , "Centre, heart" , "The centre part, as of an apple" , "Drilled rock sample" .)