Syrup, say maple, about a centilitre (7)
I believe the answer is:
treacle
'syrup' is the definition.
(I know that treacle is a type of syrup)
'maple about a centilitre' is the wordplay.
'maple' becomes 'tree' (maple is a type of tree).
'about' is an insertion indicator.
'centilitre' becomes 'cl' (standard abbreviation).
'a'+'cl'='acl'
'tree' placed around 'acl' is 'TREACLE'.
'say' is the link.
(Other definitions for treacle that I've seen before include "Thick, sticky, dark syrup" , "Syrup, molasses" , "Carroll's sisters had to live with" , "cloying stuff" , "Sweet, sticky stuff" .)