Vet observed tackling case of canker (6)
I believe the answer is:
screen
'vet' is the definition.
(both can mean to check or investigate)
'observed tackling case of canker' is the wordplay.
'observed' becomes 'seen' (synonyms).
'tackling' indicates putting letters inside (to tackle is to take possession of).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'canker' with its centre removed is 'cr'.
'seen' going around 'cr' is 'SCREEN'.
(Other definitions for screen that I've seen before include "Veil; vet" , "following 28 [SILVER]" , "Examine carefully to select or eliminate" , "Room divider" , "'It provides protection, against insects, say (6)'" .)