Motive (5)
'motive' is the definition.
(I know that reason is a type of rational motive)
This is the entire clue.
(Other definitions for reason that I've seen before include "issue" , "Sanity of senora" , "Cause, explanation" , "Justification; sanity" , "brain operations" .)
'motive' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cause that I've seen before include "Grounds" , "why some people fight" , "Make happen; something deserving support" , "A movement or charity- usually good" , "Genesis" .)