Syrup, maple perhaps, around a hundred litres (7)
I believe the answer is:
treacle
'syrup' is the definition.
(I know that treacle is a type of syrup)
'maple perhaps around a hundred litres' is the wordplay.
'maple perhaps' becomes 'tree' (maple is a type of tree).
'around' means one lot of letters goes inside another.
'hundred' becomes 'c' (Roman numeral).
'litres' becomes 'l' (abbreviation for litres).
'a'+'c'+'l'='acl'
'tree' placed around 'acl' is 'TREACLE'.
(Other definitions for treacle that I've seen before include "Saplike stuff" , "Sweet sticky syrup" , "well described by Dormouse" , "Sticky produce" , "Syrup, molasses" .)