The most common and most likely answer to this crossword clue is the 8 letter word LINIMENT
I've seen this crossword clue in The Guardian
L
I
N
I
M
E
N
T
More clues leading to the result LINIMENT
- At home this person’s invested in fast pain relief
- Reversing car in fast shows application
- Fast car reversed inside – there’s the rub
- Embrocation, one chaps wrapped in cotton fibre
- Cloth in which artist get wrapped up — it relieves the pain
- Ointment ruined linen mitt almost entirely
- Small retreats in lead-up to Easter, which 11 may use to relax 13
Buddy explains!

The straight part
We know that "it relieves the pain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it relieves the pain = liniment.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.
What does LINIMENT mean?
LINIMENT noun- A topical medical preparation intended to be rubbed into the skin with friction, as for example to relieve symptoms of arthritis.
- To apply liniment to.