A lot of investigation regarding Ohio is done for free (3,4)
I believe the answer is:
pro bono
'done for free' is the definition.
'pro bono' can be an answer for 'free' (I've seen this before). I am not certain of the 'done for' bit.
'a lot of investigation regarding ohio' is the wordplay.
'a lot of' means to remove the last letter (most of the word but not all of it).
'investigation' becomes 'probe' (probe can mean an investigation or inquiry).
'regarding' becomes 'on' (on or regarding a particular topic).
'ohio' becomes 'O' (abbreviation for Ohio).
'probe' with its last letter taken off is 'prob'.
'prob'+'on'+'o'='PRO BONO'
'is' is the link.
Can you help me to learn more?
(Other definitions for pro bono that I've seen before include "Done for the public good unpaid" , "-- -- publico (Latin)" , "Giving free services" , "with no charges" , "For the public good - ... .... publico" .)