Doctor satisfied about national importance (6)
I believe the answer is:
moment
'importance' is the definition.
(moment is a kind of importance)
'doctor satisfied about national' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'satisfied' becomes 'met' (as in 'met the conditions of his contract').
'about' means one lot of letters goes inside another.
'national' becomes 'n' (abbreviation listed in Chambers).
'met' placed around 'n' is 'ment'.
'mo'+'ment'='MOMENT'
(Other definitions for moment that I've seen before include "tick" , "Import" , "Instant - importance" , "An exact point of time" , "little while" .)