She maintains service at a high level (3,7)
I believe the answer is:
air hostess
'high level' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'she maintains service at a' is the wordplay.
I cannot quite see how this works, but
'she' could be 'tess' and 'tess' is found within the answer.
'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for air hostess that I've seen before include "Serving flyer" , "Stewardess on a plane" , "Female flight attendant" , "Female working in plane" , "high flyer" .)