Body of legislators had to back second measure (6)
I believe the answer is:
senate
'body of legislators' is the definition.
'had to back second measure' is the wordplay.
'had' becomes 'ate' (e.g. I had an ice cream).
'to back' is a charade indicator (letters next to each other) (some letters go at the back of others).
'second' becomes 's' (common abbreviation).
'measure' becomes 'en' (typographical measure).
's'+'en'='sen'
'ate' put after 'sen' is 'SENATE'.
(Other definitions for senate that I've seen before include "State assembly" , "Political body" , "Governors here" , "Legislative assembly" , "Upper council of university" .)