Small vehicle accommodating a large quantity (6)
I believe the answer is:
scalar
'quantity' is the definition.
(I know that scalar is a type of variable)
'small vehicle accommodating a large' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vehicle' becomes 'car' (car is a kind of vehicle).
'accommodating' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'a'+'l'='al'
'car' going around 'al' is 'calar'.
's'+'calar'='SCALAR'
(Other definitions for scalar that I've seen before include "With magnitude, not direction (maths)" , "(Quantity) with magnitude, not direction" , "lacking direction" , "graduated" .)