Doctor misses treacle (8)
I believe the answer is:
molasses
'treacle' is the definition.
(thesaurus)
'doctor misses' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'misses' becomes 'lasses' (lass is a kind of miss).
'mo'+'lasses'='MOLASSES'
(Other definitions for molasses that I've seen before include "Thick dark syrup made from sugar cane" , "Syrup drained from raw sugar for some lass" , "Thick dark treacle" , "Dark syrup from raw sugar" , "what refinement can bring" .)