Gap right in the middle of California causes complaint (7)
I believe the answer is:
cholera
'complaint' is the definition.
(cholera is a kind of disease)
'gap right in the middle of california' is the wordplay.
'gap' becomes 'hole' (hole is a kind of gap).
'right' becomes 'r' (common abbreviation).
'in the middle of' is an insertion indicator.
'california' becomes 'ca'.
'hole'+'r'='holer'
'holer' going within 'ca' is 'CHOLERA'.
'causes' is the link.
(Other definitions for cholera that I've seen before include "Solemn promise" , "Intestinal infection caused by contaminated water or food" , "Deadly, infectious disease" , "Acute intestinal infection from contaminated water or food" , "Infectious and often fatal Asiatic disease" .)