Old brass instrument Caroline mostly played (7)
I believe the answer is:
clarion
'old brass instrument' is the definition.
(type of horn)
'caroline mostly played' is the wordplay.
'mostly' means to remove the last letter.
'played' is an anagram indicator.
'caroline' with its final letter removed is 'carolin'.
'carolin' with letters rearranged gives 'CLARION'.
(Other definitions for clarion that I've seen before include "Clear sort of call" , "Kind of trumpet" , "Loud and clear; shrill war trumpet" , "Medieval brass instrument with a clear shrill tone" , "A loud rousing sound" .)