Speak softly when recalling spirit double (6)
I believe the answer is:
murmur
'speak softly' is the definition.
(synonyms)
'recalling spirit double' is the wordplay.
'recalling' says the letters should be written in reverse.
'spirit' becomes 'rum' (alcoholic spirit).
'double' says to double the letters.
'rum' in reverse letter order is 'mur'.
'mur' doubled is 'MURMUR'.
'when' acts as a link.
(Other definitions for murmur that I've seen before include "Little noise" , "Low background noise" , "Abnormal sound made by heart" , "Speak quietly" , "Low continuous sound" .)