Tiny tree cultivated for a long time (8)
I believe the answer is:
eternity
'long time' is the definition.
(I've seen this before)
'tiny tree cultivated' is the wordplay.
'cultivated' indicates anagramming the letters.
'tiny'+'tree'='tinytree'
'tinytree' with letters rearranged gives 'ETERNITY'.
'for a' acts as a link.
(Other definitions for eternity that I've seen before include "timelessness" , "You never come to the end of this" , "Describing ring" , "Endless period" , "Endless duration" .)