Old friend talked of spirit's shimmering effect (11)
I believe the answer is:
opalescence
'shimmering effect' is the definition.
(I've seen this before)
'old friend talked of spirit's' is the wordplay.
'old friend' becomes 'opal' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'talked of' indicates a 'sounds like' (homophone) clue.
'spirit' becomes 'essence' (essence can mean something's spirit or heart).
'essence' sounds like 'escence'.
'opal'+'escence'='OPALESCENCE'
Can you help me to learn more?
(Other definitions for opalescence that I've seen before include "Shifting colour" , "Milky iridescent quality" , "Milky iridescence" , "Shimmering effect" , "milkiness" .)