I am sharp with Mompou’s last piano piece (9)
I believe the answer is:
impromptu
'piano piece' is the definition.
(I've seen this before)
'i am sharp with mompou's last' is the wordplay.
'i am' becomes 'I'm' (contraction).
'sharp' becomes 'prompt' (I've seen this before).
'with' says to put letters next to each other.
'last' says to take the final letters.
The final letter of 'mompou' is 'u'.
'im'+'prompt'+'u'='IMPROMPTU'
(Other definitions for impromptu that I've seen before include "Offhand" , "Extempore" , "Extemporaneous speech" , "Piano piece" , "Unplanned; composition" .)