Fitting customer means endless changes (12)
I believe the answer is:
commensurate
'fitting' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'customer means endless changes' is the wordplay.
'endless' means to remove the last letter.
'changes' is an anagram indicator.
'customer'+'means'='customermeans'
'customermeans' with its last letter taken away is 'customermean'.
'customermean' is an anagram of 'COMMENSURATE'.
Can you help me to learn more?
(Other definitions for commensurate that I've seen before include "Appropriate (to)" , "Proportionate" , "In proportion" , "Corresponding with" , "equal" .)