Sample lacking internal tension is a shocker (6)
I believe the answer is:
taster
'sample' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for taster that I've seen before include "A sample to try" , "person in charge of testing" , "Brief sample" , "Small sample" , "Food tester" .)