Guard spotted protecting both ends of corridor (6)
I believe the answer is:
screen
'guard' is the definition.
(screen can mean to guard or protect)
'spotted protecting both ends of corridor' is the wordplay.
'spotted' becomes 'seen' (I've seen this before).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'both ends of' says to hollow out the word (remove centre letters) (only the letters at each end of the word).
'corridor' with its middle taken out is 'cr'.
'seen' enclosing 'cr' is 'SCREEN'.
(Other definitions for screen that I've seen before include "Blind - hide from view" , "cloak" , "not to show? That's a riddle" , "Partition; monitor" , "Hide - show - partition" .)