Derbyshire town developing leftish creed (12)
I believe the answer is:
chesterfield
'derbyshire town' is the definition.
(town in Derbyshire)
'developing leftish creed' is the wordplay.
'developing' indicates anagramming the letters (letters developed into a new form).
'leftish'+'creed'='leftishcreed'
'leftishcreed' anagrammed gives 'CHESTERFIELD'.
(Other definitions for chesterfield that I've seen before include "Well-padded sofa with two arms and a back" , "Derby town?" , "Sofa; English town" , "Sofa (with crooked spire?)" , "Padded sofa" .)