A line about Churchill's first agreement (6)
I believe the answer is:
accord
'agreement' is the definition.
(accord is a kind of agreement)
'a line about churchill's first' is the wordplay.
'line' becomes 'cord' (cord is a kind of line).
'about' means one lot of letters goes inside another.
'first' indicates taking the first letters.
The first letter of 'churchills' is 'c'.
'cord' going around 'c' is 'ccord'.
'a'+'ccord'='ACCORD'
(Other definitions for accord that I've seen before include "Consent or agreement" , "binding agreement" , "Agreement - conformity" , "Agree" , "Model" .)