Father's up, getting tedious -- that's terrible! (9)
I believe the answer is:
appalling
'terrible' is the definition.
(I've seen this before)
'father's up getting tedious' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'getting' means one lot of letters go next to another.
'tedious' becomes 'palling' (I am not sure about this - if you are sure you should believe this answer much more).
'pa' back-to-front is 'ap'.
'ap'+'palling'='APPALLING'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for appalling that I've seen before include "Terrible" , "Awful" , "Horrific" , "Shocking, outrageous" , "Shockingly bad" .)