An officer in Ypres played old instrument (8)
I believe the answer is:
psaltery
'old instrument' is the definition.
(ancient string instrument)
'an officer in ypres played' is the wordplay.
'an officer' becomes 'alt' (I am not sure about this - if you are sure you should believe this answer much more).
'in' is an insertion indicator.
'played' indicates an anagram.
'ypres' is an anagram of 'psery'.
'alt' placed into 'psery' is 'PSALTERY'.
Can you help me to learn more?
(Other definitions for psaltery that I've seen before include "string instrument of yore" , "Old plucked instrument" , "Ancient plucked instrument" , "something demanding pluck" , "Musical instrument" .)