Someone who carefully reads how a service may be charged (7)
I believe the answer is:
peruser
'someone who carefully reads' is the definition.
The definition suggests a singular noun which matches the answer.
'a service may be charged' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'service may' becomes 'ruse' (I can't justify this - if you can you should give a lot more credence to this answer).
'be charged' indicates putting letters inside (charged can mean filled with).
'per' placed around 'ruse' is 'PERUSER'.
'how' acts as a link.
Can you help me to learn more?
(Other definitions for peruser that I've seen before include "A careful reader or browser" , "One examining" , "Scanner" , "one reads" , "Rep rues meeting a reader" .)