The necessary part of the orchestra (5)
I believe the answer is:
brass
'the orchestra' is the definition.
(thesaurus)
'the necessary part' is the wordplay.
'the necessary' becomes 'br' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'part' becomes 'ass' (ass is a kind of part).
'br'+'ass'='BRASS'
'of' is the link.
Can you help me to learn more?
(Other definitions for brass that I've seen before include "An alloy; part of orchestra" , "Alloy; money" , "memorial in church" , "Front" , "Yellowish alloy" .)