Wrongly believed hotel is obliged to put on film (10)
I believe the answer is:
Here is my best explanation:
'wrongly believed' is the definition.
'hotel is obliged to put on film' is the subsidiary indication.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'is obliged to' becomes 'ought'.
'put on' means one lot of letters go next to another.
'film' becomes 'mist' (I've seen this in other clues**).
'hought' put after 'mist' is 'misthought'.