Firm around middle (4)
I believe the answer is:
core
'middle' is the definition.
(core can mean the centre of something)
'firm around' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'around' becomes 're' (regarding).
'co'+'re'='CORE'
(Other definitions for core that I've seen before include "Crux" , "Inner part" , "Central zone" , "Drilled rock sample" , "Planet's centre" .)