The most common and most likely answer to this crossword clue is the 8 letter word BUOYANCY
I've seen this crossword clue in The Daily Telegraph
B
U
O
Y
A
N
C
Y
Buddy explains!

The straight part
We know that "resistance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case resistance = buoyancy.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 BUOYANCY mean?
BUOYANCY noun- The upward force on a body immersed or partly immersed in a fluid.
- The ability of an object to stay afloat in a fluid.
- (by extension) Resilience or cheerfulness.