Gauge Conservative over interior changes (9)
I believe the answer is:
criterion
'gauge' is the definition.
(thesaurus)
'conservative over interior changes' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'changes' indicates anagramming the letters.
'interior' with letters rearranged gives 'riterion'.
'c'+'riterion'='CRITERION'
(Other definitions for criterion that I've seen before include "Standard by which something can be judged" , "Standard by which to judge" , "Judging standard" , "London theatre?" , "Standard; model" .)