Fed up, work to get rid of the cause of tension (6)
I believe the answer is:
defuse
'get rid of the cause of tension' is the definition.
I can't tell whether this definition defines the answer.
'fed up work' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'work' becomes 'use' (work can mean to use or operate).
'fed' back-to-front is 'def'.
'def'+'use'='DEFUSE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for defuse that I've seen before include "Render bomb or situation harmless" , "Remove the triggering device" , "undertake bomb disposal" , "Reduce dangerous condition, in bomb or situation" , "Make (bomb) safe" .)