Very different doctor patrols ward (6,5)
I believe the answer is:
worlds apart
'very different' is the definition.
(I've seen this before)
'doctor patrols ward' is the wordplay.
'doctor' indicates an anagram.
'patrols'+'ward'='patrolsward'
'patrolsward' is an anagram of 'WORLDS APART'.
(Other definitions for worlds apart that I've seen before include "As different as is possible" , "Separated by great distances" .)