Ultimately got free rein, mistakenly, to meddle (9)
I believe the answer is:
interfere
'to meddle' is the definition.
(to meddle with something is to interfere with it)
'ultimately got free rein mistakenly' is the wordplay.
'ultimately' says to take the final letters.
'mistakenly' indicates anagramming the letters.
The final letter of 'got' is 't'.
't'+'free'+'rein'='tfreerein'
'tfreerein' anagrammed gives 'INTERFERE'.
(Other definitions for interfere that I've seen before include "Intruder does" , "Stick one's oar in" , "Meddle, poke into" , "Come between, intervene" , "Meddle in someone else's business" .)