Trailer to run over piano initially (7)
I believe the answer is:
preview
'trailer' is the definition.
(I know that trailer can be written as preview)
'run over piano initially' is the wordplay.
'run over' becomes 'review' (I've seen this in another clue).
'piano' becomes 'p' (musical abbreviation).
'initially' means one lot of letters go next to another (some letters go before others).
'review' put after 'p' is 'PREVIEW'.
'to' is the link.
(Other definitions for preview that I've seen before include "See ahead" , "Early chance to see" , "Foretaste" , "Hint" , "Advance showing before general release" .)