A cold sea ruined what a doctor has on (8)
I believe the answer is:
caseload
'what a doctor has on' is the definition.
(caseload refers to a doctor's patients)
'a cold sea ruined' is the wordplay.
'ruined' indicates anagramming the letters.
'a'+'cold'+'sea'='acoldsea'
'acoldsea' with letters rearranged gives 'CASELOAD'.
(Other definitions for caseload that I've seen before include "Number of jobs to be handled in a given time, by a lawyer say" , "doctor's list" , "outstanding work" , "Matters to deal with" , "all doctor has to deal with?" .)