Correlation shown where moon follows rodent (5)
I believe the answer is:
ratio
'correlation' is the definition.
(I've seen this before)
'moon follows rodent' is the wordplay.
'moon' becomes 'Io' (moon of Jupiter).
'follows' is a charade indicator (letters next to each other).
'rodent' becomes 'rat' (rat is a kind of rodent).
'io' put after 'rat' is 'RATIO'.
'shown where' acts as a link.
(Other definitions for ratio that I've seen before include "Comparative measure" , "Proporation, quantitative relation" , "2:1, for example" , "five to four, perhaps" , "Proportion, relation" .)