Support film about extremely serious lady of letters (12)
I believe the answer is:
postmistress
'lady of letters' is the definition.
(woman responsible for a postal service)
'support film about' is the wordplay.
'support' becomes 'post' (post is a kind of support).
'film about' becomes 'mistress' (I can't justify this - if you can you should give a lot more credence to this answer).
'post'+'mistress'='POSTMISTRESS'
'extremely serious' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for postmistress that I've seen before include "Mail manager" , "Local head in mail service" , "Office manager" .)