Freely commercial library (2-3)

I believe the answer is:

ad-lib

But I can't explain it at all!

(Other definitions for ad-lib that I've seen before include "Depart from the text" , "Improvise" , "Off the cuff remark" , "Speak impromptu" , "Speak without preparation" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app