Supported by state before maturity, is toeing the party line (2,7)
I believe the answer is:
on message
'toeing the party line' is the definition.
(eg following a political party's line on something)
'supported by state before maturity' is the wordplay.
'supported by' becomes 'on' (something on something else is supported by it).
'state' becomes 'mess' (in a state means in a mess).
'before' means one lot of letters go next to another.
'maturity' becomes 'age' (I've seen this before).
'on'+'mess'+'age'='ON-MESSAGE'
'is' acts as a link.
(Other definitions for on message that I've seen before include "keeping to the script" , "Following official line" , "Following party line" , "Reflecting the official line" , "in accordance with policy" .)