Firm around the middle (4)
I believe the answer is:
core
'the middle' is the definition.
(core can mean the centre of something)
'firm around' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'around' becomes 're' (I can't justify this - if you can you should believe this answer much more).
'co'+'re'='CORE'
Can you help me to learn more?
(Other definitions for core that I've seen before include "Centre, heart" , "Central zone" , "Planet's centre" , "Nucleus" , "The innermost part" .)