One can never be sure if one has it (5)
I believe the answer is:
doubt
'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" .)