Indicate agreement, with nothing but excited certainty (2,5)
I believe the answer is:
no doubt
'indicate agreement with nothing but excited certainty' is the definition.
I can't tell whether this definition defines the answer.
'indicate agreement with nothing but excited' is the wordplay.
'indicate agreement' becomes 'nod' (I've seen this before).
'with' means one lot of letters goes inside another.
'nothing' becomes 'O' (looks like zero - 0).
'excited' is an anagram indicator.
'but' anagrammed gives 'ubt'.
'o' put into 'ubt' is 'oubt'.
'nod'+'oubt'='NO DOUBT'
Can you help me to learn more?
(Other definitions for no doubt that I've seen before include "Surely" , "To be sure" , "Almost certainly" , "sign of agreement" , "Bound to, unquestionably" .)