Exist, say, as a dam-builder (6)
I believe the answer is:
beaver
'a dam-builder' is the definition.
(I've seen this before)
'exist say' is the wordplay.
'exist' becomes 'be' (synonyms).
'say' becomes 'aver' (aver means to state or claim something).
'be'+'aver'='BEAVER'
'as' acts as a link.
(Other definitions for beaver that I've seen before include "Large furry rodent" , "Dam-building animal" , "Large amphibious rodent" , "Dam-builder" , "Lodge-building rodent" .)