Civic dignitary planting tree on island (8)
I believe the answer is:
alderman
'civic dignitary' is the definition.
(I've seen this before)
'tree on island' is the wordplay.
'tree' becomes 'alder' (alder tree is a kind of tree).
'on' means one lot of letters go next to another.
'island' becomes 'man' (the Isle of Man is an example).
'alder'+'man'='ALDERMAN'
'planting' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
(Other definitions for alderman that I've seen before include "Member of a local legislative body" , "In England and Wales, a former senior local councillor" , "superior member of county council" , "Civic officer" , "Local politician" .)