Declare tenor's in poor taste (6)
I believe the answer is:
attest
'declare' is the definition.
(attesting is a kind of declaring)
'tenor's in poor taste' is the wordplay.
'tenor' becomes 't'.
'in' is an insertion indicator.
'poor' indicates an anagram.
'taste' is an anagram of 'atest'.
't' put into 'atest' is 'ATTEST'.
(Other definitions for attest that I've seen before include "Bear formal witness to, give evidence" , "Provide firm evidence" , "Corroborate" , "Certify or bear witness" , "Give evidence - sounds a trial" .)