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' indicates putting letters inside.
'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" .)