Knowing sort of look: mystery's ending (5)
I believe the answer is:
leery
'knowing' is the definition.
(I've seen this before)
'sort of look mystery's ending' is the wordplay.
'sort of look' becomes 'leer'.
'ending' indicates one should take the final letters (the ending of the word).
The final letter of 'mystery' is 'y'.
'leer'+'y'='LEERY'
(Other definitions for leery that I've seen before include "In slang, wary or suspicious - reely!" , "Cunning - wary" , "Sly, cunning" , "Shy" , "Suspiciously cautious" .)