Black knight pens song for Scottish child (5)
I believe the answer is:
bairn
'scottish child' is the definition.
(I've seen this before)
'black knight pens song' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'knight' becomes 'n' (chess).
'pens' indicates putting letters inside.
'song' becomes 'air' (I've seen this before**).
'b'+'n'='bn'
'bn' enclosing 'air' is 'BAIRN'.
'for' acts as a link.
(Other definitions for bairn that I've seen before include "Wee child" , "Child (of the North!)" , "A tot of Scotch" , "Scot. child" , "A Scottish child?" .)