Turkey by long level passage (7)
I believe the answer is:
trachea
'passage' is the definition.
(I've seen this before)
'turkey by long level' is the wordplay.
'turkey' becomes 'tr' (abbreviation eg in internet addresses).
'by' says to put letters next to each other.
'long' becomes 'ache' (to long for something is to ache for it).
'level' becomes 'a' (A level is an example).
'tr'+'ache'+'a'='TRACHEA'
(Other definitions for trachea that I've seen before include "air passage" , "pipe" , "where air passes" , "airy passage" , "airway" .)