Strange spirit summoned up unnatural sound from organ (6)
I believe the answer is:
murmur
'unnatural sound from organ' is the definition.
(as in a heart murmur)
'strange spirit summoned up' is the wordplay.
'strange' becomes 'rum' (similar in meaning).
'spirit' becomes 'rum' (I've seen this before).
'summoned up' says the letters should be written backwards.
'rum'+'rum'='rumrum'
'rumrum' written backwards gives 'MURMUR'.
(Other definitions for murmur that I've seen before include "Quiet utterance" , "Speak softly or indistinctly" , "Low indistinct sound" , "Low background noise" , "Soft voices (of discontent)" .)