If (7)
'if' is the definition.
(synonyms)
This is all the clue.
(Other definitions for whether that I've seen before include "- - 'tis nobler in the mind... Shakespeare's 'Hamlet'" , "Word introduces alternative" , "He threw (anag.)" , "Whichever is the case" , "-- or no means in any event" .)
(Other definitions for only that I've seen before include "Siblingless (child)" , "Exclusive - individual" , ". . . . the Lonely (sung by Roy Orbison)" , "Solely, lone" , "Existing alone with no other, perhaps a child" .)