Firm about the middle (4)
I believe the answer is:
core
'the middle' is the definition.
(core can mean the centre of something)
'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 "The centre of an object" , "Inner part" , "The innermost part" , "Centre, heart" , "Central zone" .)