Song was first number one to be included (4)
I believe the answer is:
lied
'song' is the definition.
(lied is a kind of song)
'was first number one to be included' is the wordplay.
'was first' becomes 'led'.
'number' becomes 'i' (I is an example**).
'one to be included' is an insertion indicator.
'led' enclosing 'i' is 'LIED'.
(Other definitions for lied that I've seen before include "untrue" , "Told fibs - German song" , "Stated falsely" , "Did not speak the truth" , "gave a false impression" .)