Something painful written up in pencil, occasionally (5)
I believe the answer is:
colic
'something painful written' is the definition.
I can't tell whether this defines the answer.
'up in pencil occasionally' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' says the answer is hidden in the clue.
'ciloc' is hidden within 'pencil occasionally'.
'ciloc' reversed gives 'COLIC'.
Can you help me to learn more?
(Other definitions for colic that I've seen before include "Abdominal pain from wind or obstruction" , "Gastric pain" , "Abdominal pain suffered by babies and horses" , "gut reaction" , "Inner pain" .)