In Montmartre, a clear, sticky substance (7)
I believe the answer is:
treacle
'sticky substance' is the definition.
(I've seen this before)
'in montmartre a clear' is the wordplay.
'in' says the answer is hidden in the clue.
'TREACLE' is hidden within 'montmartre a clear'.
(Other definitions for treacle that I've seen before include "Viscous liquid" , "cloying stuff" , "Syrup, molasses" , "Carroll's sisters had to live with" , "Sticky produce" .)