This is simple.  You can say the following:

 

"You are being considered for a position which requires a security clearance.  When the Government reviews your application, they will look for the following:

 

Criminal conviction resulting in incarceration for a period of one year or more
Current unlawful use of or addiction to a controlled substance
Determined to be mentally incompetent by a mental health professional approved by DoD
Discharge or dismissal from the Armed Forces under dishonorable conditions
Unwillingness to surrender a foreign passport

 

Otherwise, the most common reasons for clearance denial are serious repeated financial problems, intentional false statements in connection with a clearance investigation, recent illegal drug involvement, repeated alcohol abuse, and a pattern of criminal conduct or rule violation. For many people these issues can be mitigated, if presented properly during a security interview or in response to a Letter of Intent to deny clearance.

 

Wihout going into detail, do any of these pertain to you?  If you'd like to discuss in further detail, I can put you in touch with our FSO."