Legal bill's going ahead with small changes (7)
I believe the answer is:
adjusts
'changes' is the definition.
(adjusting is a kind of changing)
'legal bill's going ahead with small' is the wordplay.
'legal' becomes 'just' (I can't justify this - if you can you should give a lot more credence to this answer).
'bill' becomes 'ad' (advertisement stuck on wall).
'going ahead' says to put letters next to each other (some letters go in front of others).
'with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'just' after 'ad' is 'adjust'.
'adjust'+'s'='ADJUSTS'
Can you help me to learn more?
(Other definitions for adjusts that I've seen before include "suits" , "Alters to regulate" .)