Saw doctor with ill tot (5)
I believe the answer is:
motto
'saw' is the definition.
(thesaurus)
'doctor with ill tot' is the wordplay.
'doctor' becomes 'mo' (medical officer**).
'with' means one lot of letters goes inside another.
'ill' indicates an anagram.
'tot' anagrammed gives 'ott'.
'mo' placed around 'ott' is 'MOTTO'.
(Other definitions for motto that I've seen before include "Catchphrase" , "Brief guiding rule" , "Saw" , "Repeated theme in music" , "Catchcry, slogan" .)