Pour drink trapping small section of air (7)
I believe the answer is:
descant
'section of air' is the definition.
'descant' can be an answer for 'air' (descant is a kind of air). I am not certain of the 'section of' bit.
'pour drink trapping small' is the wordplay.
'pour' becomes 'decant' (synonyms).
'drink' indicates putting letters inside.
'trapping small' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'decant' placed around 's' is 'DESCANT'.
Can you help me to learn more?
(Other definitions for descant that I've seen before include "Independent melody sung above a basic melody" , "Accompaniment added above a basic melody" , "Upper melody" , "Ornamental melody added above a theme" , "Highest range (eg of a 10)" .)