Follow up on unknown trouble with a movie (8)
I believe the answer is:
godzilla
'movie' is the definition.
(Japanese film franchise)
'follow up on unknown trouble' is the wordplay.
'follow' becomes 'dog' (dog can mean to follow or torment).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'on' means one lot of letters go next to another.
'unknown' becomes 'z' (algebra).
'trouble' becomes 'illa' (I am not sure about this - if you are sure you should believe this answer much more).
'dog' reversed gives 'god'.
'god'+'z'+'illa'='GODZILLA'
'with a' acts as a link.
(Other definitions for godzilla that I've seen before include "Japanese film monster" , "movie monster" .)