South America reduced protection applied to old cactus (7)
I believe the answer is:
saguaro
'cactus' is the definition.
(saguaro is a kind of cactus)
'south america reduced protection applied to old' is the wordplay.
'south america' becomes 'SA' (abbreviation).
'reduced' means to remove the last letter.
'protection' becomes 'guard' (I've seen this before).
'applied to' means one lot of letters go next to another.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'guard' with its final letter taken away is 'guar'.
'sa'+'guar'+'o'='SAGUARO'
(Other definitions for saguaro that I've seen before include "Large branching cactus" , "Branched cactus" , "Huge cactus" , "Giant type of cactus" .)