At the weekend, oil is spilt in quantity (4)
I believe the answer is:
kilo
'quantity' is the definition.
(kilo is a kind of quantity)
'at the weekend oil is spilt' is the wordplay.
'at' means one lot of letters go next to another.
'the weekend' becomes 'k' (I've seen this in other clues).
'is spilt' is an anagram indicator.
'oil' is an anagram of 'ilo'.
'k'+'ilo' is 'KILO'.
'in' acts as a link.
(Other definitions for kilo that I've seen before include "more than two pounds" , "Lewis Carroll heroine" , "Metrical unit of weight" , "Prefix for thousand" , "Unit of weight (abbr)" .)