Composition old soldiers put in proportion (8)
I believe the answer is:
oratorio
'composition' is the definition.
(I've seen this before)
'old soldiers put in proportion' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'soldiers' becomes 'or' (Other Rank).
'put in' indicates putting letters inside.
'proportion' becomes 'ratio' (I've seen this before).
'or' going inside 'ratio' is 'ratorio'.
'o'+'ratorio'='ORATORIO'
(Other definitions for oratorio that I've seen before include "Sacred musical piece" , "Musical composition for voices, chorus and orchestra with biblical theme" , "Work for many performers" , "Eg, The Seasons (Haydn)" , "Music for voices and orchestra with biblical theme" .)