Footing bill, initially lacking change (5)
I believe the answer is:
basis
'footing' is the definition.
(synonyms)
'bill initially lacking change' is the wordplay.
'initially' indicates taking the first letters.
'lacking change' becomes 'asis' ('as is' can mean as something currently is).
The first letter of 'bill' is 'b'.
'b'+'asis'='BASIS'
(Other definitions for basis that I've seen before include "Foundation or support, or a main principle" , "First principle" , "grounds" , "Principle on which a theory is founded" , "Groundwork, foundation" .)