Age of tree in ground? Time unknown (8)
I believe the answer is:
eternity
'age' is the definition.
(I've seen this before)
'tree in ground? time unknown' is the wordplay.
'ground?' indicates anagramming the letters ('grind' the letters into a new form).
'time' becomes 't' (abbreviation).
'unknown' becomes 'y' (algebra).
'tree'+'in'='treein'
'treein' with letters rearranged gives 'eterni'.
'eterni'+'t'+'y'='ETERNITY'
'of' acts as a link.
(Other definitions for eternity that I've seen before include "Time without end" , "Age" , "Jean-Paul --, French author" , "Forever and ever" , "Type of ring" .)