Bill's partner with new diet consumed 84 starters and felt better as a result (9)

'felt better as a result' is the definition.

'bill's partner with new diet consumed 84 starters' is the wordplay.
'bill's partner' becomes 'ben' (bill and ben the flowerpot men).
'with' means one lot of letters go next to another.
'new' is an anagram indicator.
'consumed' means one lot of letters goes inside another.
'84 starters' becomes 'ef' (the first letters of Eighty-Four).
'diet' anagrammed gives 'ited'.
'ited' going around 'ef' is 'efited'.
'ben'+'efited'='BENEFITED'

'and' is the link.