Something to revolve around a half-dozen turns (4)
I believe the answer is:
axis
'something to revolve around' is the definition.
(eg a planet revolves around its axis)
'a half-dozen turns' is the wordplay.
'half-dozen' becomes 'six' (I've seen this before).
'turns' says the letters should be written in reverse.
'six' backwards is 'xis'.
'a'+'xis'='AXIS'
(Other definitions for axis that I've seen before include "Centre about which something turns" , "all revolves around this" , "x or y line on a graph" , "Imaginary line around which object rotates" , "Pivot, political alliance" .)