Irrational former wife changes meter (7)
I believe the answer is:
extreme
'irrational' is the definition.
(thesaurus)
'former wife changes meter' is the wordplay.
'former wife' becomes 'ex' (an ex-wife).
'changes' is an anagram indicator.
'meter' with letters rearranged gives 'treme'.
'ex'+'treme'='EXTREME'
(Other definitions for extreme that I've seen before include "very strict" , "Ultimate" , "Exceptional - radical" , "not least" , "Immoderate" .)