Worries about filling bag up (5)
I believe the answer is:
cares
'worries' is the definition.
(I've seen this before)
'about filling bag up' is the wordplay.
'about' becomes 're' (regarding**).
'filling' is an insertion indicator.
'bag' becomes 'sac' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'sac' written backwards gives 'cas'.
're' inserted inside 'cas' is 'CARES'.
(Other definitions for cares that I've seen before include "Has concerns or interest" , "Worries, anxieties" , "Feels interest or concern" , "Minds" .)