Tenner originally put in bucket by coercion, unquestionably (3,7)
I believe the answer is:
for certain
'unquestionably' is the definition.
Both the definition and answer 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 can't justify this - if you can you should believe this answer much more).
'put in' indicates putting letters inside.
'bucket' becomes 'rain' (eg bucketing down means raining).
'by' means one lot of letters go next to another.
'coercion' becomes 'force' (I've seen this before).
't' going within 'rain' is 'rtain'.
'rtain' 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" .)