Correspondence course's leader wearing a tie (6)
I believe the answer is:
accord
'correspondence' is the definition.
(thesaurus)
'course's leader wearing a tie' is the wordplay.
'leader' indicates taking the first letters.
'wearing' means one lot of letters goes inside another.
'tie' becomes 'cord' (cording is a kind of tiing**).
The initial letter of 'courses' is 'c'.
'a'+'cord'='acord'
'c' put within 'acord' is 'ACCORD'.
(Other definitions for accord that I've seen before include "Myra -, pianist; Rudolf -, Nazi" , "Agree" , "Official treaty" , "'Agreement, treaty (6)'" , "Official agreement; harmony" .)