Uncertainty about cases is widespread (4)
I believe the answer is:
rife
'widespread' is the definition.
(rife can mean widespread or universal)
'uncertainty about cases' is the wordplay.
'uncertainty' becomes 'if' (if can mean a condition or uncertainty).
'about' becomes 're' (regarding).
'cases' is an insertion indicator.
'if' placed inside 're' is 'RIFE'.
'is' acts as a link.
(Other definitions for rife that I've seen before include "is everywhere" , "raging" , "Widespread sort of fire" , "Very common or frequently occurring" , "Overabundant" .)