Sky all around circling on for ages (5)
I believe the answer is:
yonks
'for ages' is the definition.
(I've seen this before)
'sky all around circling on' is the wordplay.
'all around' shows that the letters should be reversed in order.
'circling' indicates putting letters inside (circling can mean going around or containing).
'sky' written backwards gives 'yks'.
'yks' going around 'on' is 'YONKS'.
(Other definitions for yonks that I've seen before include "Ages (informal)" , "Slangily, a long time" , "An age" , "Slang for a long time, ages" , "Many years (slang)" .)