Spaniard's so long collecting silver pieces (7)
I believe the answer is:
adagios
'pieces' is the definition.
(adagio is a kind of piece)
'spaniard's so long collecting silver' is the wordplay.
I cannot quite understand how this works, but
'silver' could be 'ag' (Ag is the chemical symbol for silver) and 'ag' is found in the answer.
'so long' could be 'adios' (synonyms) and 'adios' is found within the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for adagios that I've seen before include "Slow and graceful passages of music" , "Movements in slow time" .)