Unreasonable partner once fiddled with meter (7)
I believe the answer is:
extreme
'unreasonable' is the definition.
(thesaurus)
'partner once fiddled with meter' is the wordplay.
'partner once' becomes 'ex' (an ex-partner).
'fiddled with' indicates an anagram.
'meter' with letters rearranged gives 'treme'.
'ex'+'treme'='EXTREME'
(Other definitions for extreme that I've seen before include "Far from moderate" , "Anything but moderate" , "Very intense - way-out" , "Immoderate" , "Most distant" .)