Doctor to fit new theme (5)
I believe the answer is:
motif
'theme' is the definition.
(I know that motif is a type of idea)
'doctor to fit new' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'to' is a charade indicator (letters next to each other).
'new' is an anagram indicator.
'fit' is an anagram of 'tif'.
'mo'+'tif'='MOTIF'
(Other definitions for motif that I've seen before include "Shape" , "Recurring theme in musical work" , "Recurrent theme as in musical work" , "Logo -- pattern" , "Unifying idea in an artistic work" .)