Tree illuminated, one at back of church (6)
I believe the answer is:
litchi
'tree' is the definition.
(litchi is a kind of tree)
'illuminated one at back of church' is the wordplay.
'illuminated' becomes 'lit' ('lit' can be a synonym of 'illuminated').
'one' becomes 'i' (Roman numeral).
'at back of' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'i' after 'ch' is 'chi'.
'lit'+'chi'='LITCHI'
(Other definitions for litchi that I've seen before include "Asian tree" , "Edible Chinese fruit" , "piece of fruit" , "Chinese tree; its fruit" .)