To some extent, isn't It a niche film? (7)
I believe the answer is:
titanic
'film?' is the definition.
(I've seen this before)
'to some extent isn't it a niche' is the wordplay.
'to some extent' says the answer is hidden in the clue (I've seen 'extent of' mean this (I've seen this in other clues)).
'TITANIC' can be found hidden inside 'isnt it a niche'.
(Other definitions for titanic that I've seen before include "Doomed liner found in attic" , "Film success (1997) - ship disaster (1912)" , "Ill-fated liner" , "Hugely strong (but lost!)" , "Ship sunk by iceberg" .)