Study about very old county (5)
I believe the answer is:
devon
'county' is the definition.
(I've seen this before)
'study about very old' is the wordplay.
'study' becomes 'den' (I've seen this before).
'about' indicates putting letters inside.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'v'+'o'='vo'
'den' going around 'vo' is 'DEVON'.
(Other definitions for devon that I've seen before include "Raleigh's county" , "part of England?" , "Ilfracombe's county" , "County in SW England" , "Clotted-cream county" .)