Guard observed protecting credit (6)
I believe the answer is:
screen
'guard' is the definition.
(screen can mean to guard or protect)
'observed protecting credit' is the wordplay.
'observed' becomes 'seen' (synonyms).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'credit' becomes 'cr' (abbreviation).
'seen' enclosing 'cr' is 'SCREEN'.
(Other definitions for screen that I've seen before include "Examine for suitability, sift" , "'It provides protection, against insects, say (6)'" , "Veil; vet" , "Investigate (someone) for suitability; conceal" , "Test or examine for the presence of disease or infection" .)