State benefit is thoroughly reasonable, we're told (7)
I believe the answer is:
welfare
'state benefit' is the definition.
'welfare' can be an answer for 'benefit' (types of social security). I'm not certain of the 'state' bit.
I cannot understand how the rest of the clue works.
(Other definitions for welfare that I've seen before include "Well-being" , "Financial support for those in need" , "Governmental provision of economic assistance" , "Contented state of health and happiness" , "Social provision by state" .)