Cross priest sent back holly (4)
I believe the answer is:
ilex
'holly' is the definition.
(I've seen this before)
'cross priest sent back' is the wordplay.
'cross' becomes 'x'.
'priest' becomes 'eli' (I've seen this before).
'sent back' says the letters should be written in reverse.
'x'+'eli'='xeli'
'xeli' back-to-front is 'ILEX'.
(Other definitions for ilex that I've seen before include "common holly" , "The holly family of trees" , "Genus including holly" , "The holm-oak" , "Genus of trees that includes hollies" .)