Combine to replant giant tree (9)
I believe the answer is:
integrate
'combine' is the definition.
(integrating is a kind of combining)
'replant giant tree' is the wordplay.
'replant' indicates an anagram (I've seen this in other clues).
'giant'+'tree'='gianttree'
'gianttree' with letters rearranged gives 'INTEGRATE'.
'to' acts as a link.
(Other definitions for integrate that I've seen before include "Combine into a whole, unite" , "Organise a get-together" , "Get together" , "Incorporate into the main group" , "Join" .)