Hiatus hernia finally checked by doctor (3)
I believe the answer is:
gap
'hiatus' is the definition.
(I've seen this in another clue)
'hernia finally checked by doctor' is the wordplay.
'finally' says to take the final letters.
'checked by' is an insertion indicator.
'doctor' becomes 'gp' (general practitioner).
The last letter of 'hernia' is 'a'.
'a' placed into 'gp' is 'GAP'.
(Other definitions for gap that I've seen before include "Interval, break" , "Break in continuity" , "Cleft" , "A break or opening as in a hedge" , "Narrow space" .)