Unknowns (3)
I believe the answer is:
ifs
'unknowns' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ifs that I've seen before include "Conditional conjunctions" , "If . . . and ands were pots and pans" , "Conditions or suppositions, may join with buts" , "No ...,ands or buts, no argument allowed" , "Suppositions, may be accompanied by buts" .)