The most common and most likely answer to this crossword clue is the 5 plus 2 plus 7 plus 4 plus 5 letter word BLOOD IS THICKER THAN WATER
I've seen this crossword clue in The Guardian
B
L
O
O
D
I
S
T
H
I
C
K
E
R
T
H
A
N
W
A
T
E
R
Buddy explains!
The straight part
We know that "theory of relativity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case theory of relativity = blood is thicker than water.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.