Doctor! Cheer once to show consistency! (9)
I believe the answer is:
coherence
'consistency' is the definition.
(I've seen this before)
'doctor cheer once' is the wordplay.
'doctor' indicates anagramming the letters.
'cheer'+'once'='cheeronce'
'cheeronce' with letters rearranged gives 'COHERENCE'.
'to show' is the link.
(Other definitions for coherence that I've seen before include "Sticking together" , "Logical consistency" , "union" , "sense of unity" , "ability to talk sense" .)