Foot bill initially after confirmation from setter (4)
I believe the answer is:
iamb
'foot' is the definition.
(type of metrical foot in poetry)
'bill initially after confirmation from setter' is the wordplay.
'initially' indicates taking the first letters.
'after' is a charade indicator (letters next to each other).
'confirmation from setter' becomes 'iam' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'bill' is 'b'.
'b' after 'iam' is 'IAMB'.
Can you help me to learn more?
(Other definitions for iamb that I've seen before include "Part of a pentameter?" , "Short-long metrical foot" , "Poet's foot" , "Metrical unit" , "Line of verse" .)