Genesis, unfashionable band (6)
I believe the answer is:
outset
'genesis' is the definition.
(thesaurus)
'unfashionable band' is the wordplay.
'unfashionable' becomes 'out' ('out' can be a synonym of 'unfashionable').
'band' becomes 'set' ('set' can be a synonym of 'band').
'out'+'set'='OUTSET'
(Other definitions for outset that I've seen before include "origin" , "Opening stage" , "Fruit" , "Inception" , "Beginning, start" .)