Demanding different results, serious failings initially concealed (9)
I believe the answer is:
stressful
'demanding' is the definition.
'different results serious failings initially concealed' is the wordplay.
'different' indicates an anagram.
'initially' indicates taking the first letters.
'concealed' means one lot of letters goes inside another.
The initial letters of 'serious failings' is 'sf'.
'results' anagrammed gives 'stresul'.
'stresul' placed around 'sf' is 'STRESSFUL'.
(Other definitions for stressful that I've seen before include "Creating tension" , "Very hard on the nerves" , "under great pressure" , "Causing mental tension" , "tense" .)