Elders possibly seen in Montmartre establishment (5)
I believe the answer is:
trees
'elders possibly' is the definition.
(elder is a kind of tree)
'in montmartre establishment' is the wordplay.
'in' indicates a hidden word.
'TREES' is hidden within 'montmartre establishment'.
'seen' acts as a link.
(Other definitions for trees that I've seen before include "Tall, perennial, woody plants" , "Woods" , "Ashes, perhaps?" , "Tall, perennial, woody plant" , "Elms, pines etc" .)