Crossword Genius

One can never be sure if one has it (5)

Ross

I believe the answer is:

doubt

I'm a little stuck... Click here to teach me more about this clue!

'it' is the definition.
The definition suggests a singular noun which matches the answer.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for doubt that I've seen before include "Uncertainty about the truth" , "Question the truth or accuracy of" , "Disbelieve (someone)" , "Distrust" , "Lack of conviction" .)

I've seen this clue in The Globe and Mail.
Want a hint initially instead of a full solution? Install my app