Source of decoration using cigar in an unusual way (5,5)
I believe the answer is:
icing sugar
'source of decoration' is the definition.
(means of decorating a cake)
'using cigar in an unusual way' is the wordplay.
'in an unusual way' indicates an anagram.
'using'+'cigar'='usingcigar'
'usingcigar' anagrammed gives 'ICING SUGAR'.
(Other definitions for icing sugar that I've seen before include "Sweet powder used on cakes etc" , "Sweet stuff used for cake topping" , "Cake covering" .)