Half-a-dozen developing true goodness (6)
I believe the answer is:
virtue
'goodness' is the definition.
(virtue is a kind of goodness)
'half-a-dozen developing true' is the wordplay.
'half-a-dozen' becomes 'vi' (I've seen this in other clues).
'developing' is an anagram indicator.
'true' is an anagram of 'rtue'.
'vi'+'rtue'='VIRTUE'
(Other definitions for virtue that I've seen before include "High-mindedness" , "Rectitude" , "Maybe hope" , "Admirable feature" , "''....is it's own reward'' (6)" .)