Some clientele mentioned what is essential to them (7)
I believe the answer is:
element
'essential to them' is the definition.
I can't judge whether this defines the answer.
'some clientele mentioned what' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ELEMENT' is hidden in the letters of 'clientele mentioned what'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for element that I've seen before include "One of the periodic table" , "Singular ingredient" , "Chemical building block" , "piece" , "Most appropriate situation" .)