Maybe a man can't sing this high part (7)
I believe the answer is:
descant
'maybe a man can't sing this high part' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'man can't sing this high part' is the wordplay.
'man' becomes 'de' (I can't explain this - if you can you should give a lot more credence to this answer).
'this high' indicates taking the first letters (I've seen 'highest of' mean this (top letter - down clues only)).
'part' indicates putting letters inside (the inserted letters split open or part the word).
The first letter of 'sing' is 's'.
'de'+'cant'='decant'
'decant' enclosing 's' is 'DESCANT'.
Can you help me to learn more?
(Other definitions for descant that I've seen before include "Treble melody above the main melody" , "Melody sung or played above a basic melody" , "Highest range (eg of a 10)" , "Sung variation" , "Upper melody" .)