The most common and most likely answer to this crossword clue is the 8 plus 7 letter word RELATIVE DENSITY
I've seen this crossword clue in The Daily Telegraph
R
E
L
A
T
I
V
E
D
E
N
S
I
T
Y
More clues leading to the result RELATIVE DENSITY
- Weight ratio needs revision in Einstein's theory
- Specifically what makes blood thicker than water?
- Specifically, what makes blood thicker than water?
- Order daily entries — vet’s record of weight against a standard
- Why blood is thicker than water?
- It may prove that blood is thicker than water.
- Gravity specifically averted senility somehow
- Measure the number of aunts and uncles per house!
Buddy explains!
The straight part
We know that "Gravity specifically" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gravity specifically = relative density.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.