Number of grains behind you (8)
I believe the answer is:
thousand
'number' is the definition.
(I know that thousand is a type of large integer)
'grains behind you' is the wordplay.
'grains' becomes 'sand' (sand is made up of grains).
'behind' means one lot of letters go next to another.
'you' becomes 'thou' (historical second person pronoun).
'sand' put after 'thou' is 'THOUSAND'.
'of' acts as a link.
(Other definitions for thousand that I've seen before include "-- Island dressing" , "See 1 down" , "Figure" , "Roman numeral M" , "a couple of monkeys in total?" .)