Years and years working in flipping outer space! (5)
I believe the answer is:
yonks
'years and years' is the definition.
(I've seen this before)
'working in flipping outer space' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'in' indicates putting letters inside.
'flipping' shows that the letters should be reversed in order.
'outer space' becomes 'sky' (space is up in the sky. I am not sure about the 'outer' bit.).
'sky' written backwards gives 'yks'.
'on' inserted into 'yks' is 'YONKS'.
Can you help me to learn more?
(Other definitions for yonks that I've seen before include "eternity it seems!" , "Long time (informal)" , "Ages (informal)" , "An age" , "Many years (slang)" .)