Boast made by head of chorus line (4)
I believe the answer is:
crow
'boast' is the definition.
(crow is a kind of boast)
'by head of chorus line' is the wordplay.
'by' means one lot of letters go next to another.
'head of' says to take the initial letters.
'line' becomes 'row' (row is a kind of line**).
The initial letter of 'chorus' is 'c'.
'c' put next to 'row' is 'CROW'.
'made' is the link.
(Other definitions for crow that I've seen before include "One joins others in a murder" , "one often getting stoned!" , "Make sound of cock - bird" , "Ugly old woman" , "Black bird; brag" .)