Cross priest put up holly (4)
I believe the answer is:
ilex
'holly' is the definition.
(I've seen this before)
'cross priest put up' is the wordplay.
'cross' becomes 'x'.
'priest' becomes 'eli' (I've seen this before).
'put up' is a reversal indicator.
'x'+'eli'='xeli'
'xeli' reversed gives 'ILEX'.
(Other definitions for ilex that I've seen before include "The holm-oak" , "common holly" , "'Tree group, includes holly (4)'" , "Holly, perhaps" , "Tree of the genus that includes holly" .)