Sour mash occasionally creates problems (4)
I believe the answer is:
sums
'problems' is the definition.
(arithmetic problems)
'sour mash occasionally' is the wordplay.
'occasionally' means one should take alternating letters.
'sour'+'mash'='sourmash'
The alternate letters of 'sourmash' are 'SUMS'.
'creates' is the link.
(Other definitions for sums that I've seen before include "Amounts of money" , "Exercises in arithmetic" , "Calculations" , "Quantities obtained by addition" .)