Oh dash - the piano's covered in terrible rust! (5)
I believe the answer is:
spurt
'oh dash' is the definition.
I can't tell whether this defines the answer.
'piano's covered in terrible rust' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'covered in' indicates putting letters inside.
'terrible' is an anagram indicator.
'rust' is an anagram of 'surt'.
'p' going into 'surt' is 'SPURT'.
'the' is the link.
Can you help me to learn more?
(Other definitions for spurt that I've seen before include "Gush out suddenly" , "Short spell of increased speed" , "Gush; sudden burst" , "Jet, gush" , "A short burst of activity" .)