The most common and most likely answer to this crossword clue is the 10 letter word CO-CODAMOL
I've seen this crossword clue in The Guardian
C
O
-
C
O
D
A
M
O
L
Buddy explains!
The straight part
We know that "what can relieve one’s pain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what can relieve one’s pain = co-codamol.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.