It's sensible if one direction is the opposite direction (4)
I believe the answer is:
wise
'it's sensible' is the definition.
(I've seen this in another clue)
'one direction is the opposite direction' is the wordplay.
'one direction' becomes 'w' (W is an example).
'the opposite direction' becomes 'e' (E is an example. I am not sure about the 'opposite' bit.).
'w'+'is'+'e'='WISE'
'if' acts as a link.
Can you help me to learn more?
(Other definitions for wise that I've seen before include "Old and knowledgable" , "Old way" , "` `A word to the .... is enough' '" , "Discreet" , "comedian" .)