Mostly unrestricted speed in run (7)
I believe the answer is:
operate
'run' is the definition.
(synonyms)
'mostly unrestricted speed' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'unrestricted' becomes 'open' (similar in meaning).
'speed' becomes 'rate' (the speed at which something happens).
'open' with its last letter removed is 'ope'.
'ope'+'rate'='OPERATE'
'in' is the link.
(Other definitions for operate that I've seen before include "Perform surgery or run a machine" , "Cut - run" , "Act in a theatre" , "Work - perform surgery" , "Function; run" .)