Sit fast, regularly dismissing uncertainties (3)
I believe the answer is:
ifs
'uncertainties' is the definition.
(an if is an uncertainty or something in doubt)
'sit fast regularly dismissing' is the wordplay.
'regularly dismissing' indicates alternate letters (letters removed at regular intervals).
'sit'+'fast'='sitfast'
The alternate letters of 'sitfast' are 'IFS'.
(Other definitions for ifs that I've seen before include "Suppositions, may be accompanied by buts" , "Conditional conjunctions" , "No ...,ands or buts, no argument allowed" , "If . . . and ands were pots and pans" , "Conditions or suppositions, may join with buts" .)