Current unit in van wanting leader on base (6)
I believe the answer is:
ampere
'current unit' is the definition.
(ampere is a kind of current unit)
'van wanting leader on base' is the wordplay.
'van' becomes 'camper' (camper van).
'wanting leader' means to remove the first letter.
'on' means one lot of letters go next to another.
'base' becomes 'e' (Euler's number - base of natural logarithms).
'camper' with its initial letter taken off is 'amper'.
'amper'+'e'='AMPERE'
'in' is the link.
(Other definitions for ampere that I've seen before include "Solenoid inventor" , "Current measure" , "Unit of electrical current" , "Current analyst" , "Basic SI unit of electric current" .)