Tenner originally put in bucket by coercion, unquestionably (3,7)
I believe the answer is:
for certain
'unquestionably' is the definition.
Both the answer and definition are adverbs. Maybe they are linked in a way I don't understand?
'tenner originally put in bucket by coercion' is the wordplay.
'tenner originally' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'put in' means one lot of letters goes inside another.
'bucket' becomes 'rain' (eg bucketing down means raining).
'by' says to put letters next to each other.
'coercion' becomes 'force' (I've seen this in another clue).
't' going into 'rain' is 'rtain'.
'rtain' put after 'force' is 'FOR CERTAIN'.
Can you help me to learn more?
(Other definitions for for certain that I've seen before include "Assuredly" , "surely" , "definitely" , "without a doubt" .)