Available money is about right (4)
I believe the answer is:
free
'available' is the definition.
(similar in meaning)
'money is about right' is the wordplay.
'money' becomes 'fee' ().
'is about' means one lot of letters goes inside another.
'right' becomes 'r' (abbreviation).
'fee' placed around 'r' is 'FREE'.
(Other definitions for free that I've seen before include "See 8" , "Not confined" , "sovereign" , "unemployed" , "Rescue" .)