Old men interrupting relation's composition (8)
I believe the answer is:
oratorio
'composition' is the definition.
(I've seen this before)
'old men interrupting relation's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'men' becomes 'or' (I've seen this in other clues).
'interrupting' means one lot of letters goes inside another.
'relation' becomes 'ratio' (ratio is a kind of relation**).
'or' placed within 'ratio' is 'ratorio'.
'o'+'ratorio'='ORATORIO'
(Other definitions for oratorio that I've seen before include "Composition for voices and orchestra, often with biblical theme" , "Handel's Messiah, for example" , "Religious musical drama, unstaged" , "Musical setting for a religous text" , "Musical composition usually with biblical theme (8)" .)