Voter -- one that tweets university in agreement (11)
I believe the answer is:
constituent
'voter' is the definition.
(synonyms)
'one that tweets university in agreement' is the wordplay.
'one that tweets' becomes 'tit' (songbird).
'university' becomes 'U' (abbreviation as in UMass).
'in' indicates putting letters inside.
'agreement' becomes 'consent' (consent is a kind of agreement).
'tit'+'u'='titu'
'titu' inserted inside 'consent' is 'CONSTITUENT'.
(Other definitions for constituent that I've seen before include "voter" , "local elector" , "Person voting" , "One voting for an MP" , "Integral" .)