do custom clearance or do clearance of custom?
I am an import staff, sometime I confuse between "do custom clearance" or " do clearance of custom".
Does anybody help me?

Question

do custom clearance or do clearance of custom?
I am an import staff, sometime I confuse between "do custom clearance" or " do clearance of custom". 
Does anybody help me?

Answer

I think it actually should be "customs clearance", or "clearance from customs".
"We need to get customs clearance."
We first need to get clearance from customs."

"Customs" is an authority or agency. See https://en.wikipedia.org/wiki/Customs . 
"Custom" is an adjective with an unrelated meaning.