Worry principally about endless evil (3)
I believe the answer is:
ail
'worry' is the definition.
(I've seen this before)
'principally about endless evil' is the wordplay.
'principally' indicates taking the first letters.
'endless' means to remove the last letter.
'evil' becomes 'ill' (as in doing people ill).
The initial letter of 'about' is 'a'.
'ill' with its last letter removed is 'il'.
'a'+'il'='AIL'
(Other definitions for ail that I've seen before include "Be the worse for wear" , "Be unwell, suffer" , "Cause to feel indisposed" , "Be ill or unwell" , "Become sick" .)