Think rationally about a relative (6)
I believe the answer is:
reason
'think rationally' is the definition.
'about a relative' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'a relative' becomes 'son' (son is a kind of relative).
'rea'+'son'='REASON'
(Other definitions for reason that I've seen before include "Motivation or cause" , "Purpose" , "Defence" , "Explanation; logic" , "Cause to perform" .)