Significant space in medical speciality (7)
I believe the answer is:
eminent
'significant' is the definition.
(eminent can mean significant or important)
'space in medical speciality' is the wordplay.
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
'medical speciality' becomes 'ent' (abbreviation for Ear, Nose and Throat).
'em'+'in'+'ent'='EMINENT'
(Other definitions for eminent that I've seen before include "Important" , "Famous and admired" , "Noted" , "Lofty; famous" , "Outstanding or distinguished (7)" .)