Many years on in heaven, looking back (5)
I believe the answer is:
yonks
'many years' is the definition.
(I've seen this before)
'on in heaven looking back' is the wordplay.
'in' indicates putting letters inside.
'heaven' becomes 'sky' (I've seen this in another clue).
'looking back' shows that the letters should be reversed in order.
'sky' reversed gives 'yks'.
'on' inserted inside 'yks' is 'YONKS'.
(Other definitions for yonks that I've seen before include "eternity it seems!" , "A very long time (colloq.)" , "Donkey's years!" , "Ages (informal)" , "An age" .)