Explanation making a relation look silly (9)
I believe the answer is:
rationale
'explanation' is the definition.
(rationale is a kind of explanation)
'a relation look silly' is the wordplay.
'look silly' is an anagram indicator (I've seen this in other clues).
'a'+'relation'='arelation'
'arelation' with letters rearranged gives 'RATIONALE'.
'making' is the link.
(Other definitions for rationale that I've seen before include "Motivation" , "explanation" , "Reasoned exposition" , "Good reason" , "Logic, reasoning" .)