Malodour within old English instrument (4)
I believe the answer is:
oboe
'instrument' is the definition.
(oboe is a kind of instrument)
'malodour within old english' is the wordplay.
'malodour within' becomes 'ob' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'old english' becomes 'oe'.
'ob'+'oe'='OBOE'
Can you help me to learn more?
(Other definitions for oboe that I've seen before include "issuer of notes" , "music producer" , "Instrument used in 1d" , "Cor anglais in its family" , "it's blown" .)