Investing own money in bad film is common (10)
I believe the answer is:
downmarket
'common' is the definition.
(downmarket means common or unsophisticated)
'investing own money in bad film' is the wordplay.
'investing' indicates putting letters inside (inserted letters are taken in or invested).
'money' becomes 'mark' (I have seen 'Old money' mean 'mark' so perhaps 'money' could also mean 'mark').
'in bad film' becomes 'det' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'own'+'mark'='ownmark'
'ownmark' placed inside 'det' is 'DOWNMARKET'.
'is' is the link.
Can you help me to learn more?
(Other definitions for downmarket that I've seen before include "Budget" , "lower class" , "with less prestige" .)