Dam constructor’s extremely brittle state (6)
I believe the answer is:
beaver
'dam constructor's' is the definition.
(beavers build dams)
'extremely brittle state' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'state' becomes 'aver' (both can mean to declare).
'brittle' with its centre removed is 'be'.
'be'+'aver'='BEAVER'
(Other definitions for beaver that I've seen before include "See 22" , "Animal with soft fur, builds dams" , "Brown, furry rodent" , "Broad-tailed rodent" , "Small animal that builds dams on rivers" .)