Reformed pioneers sang about the virgin birth (15)
I believe the answer is:
parthenogenesis
'virgin birth' is the definition.
(I've seen this before)
'reformed pioneers sang about the' is the wordplay.
'reformed' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'pioneers'+'sang'='pioneerssang'
'pioneerssang' is an anagram of 'parnogenesis'.
'parnogenesis' placed around 'the' is 'PARTHENOGENESIS'.
(Other definitions for parthenogenesis that I've seen before include "virgin birth" , "Way of making young" , "Means to reproduce" .)