Hard to supplant working NASA center (7)
I believe the answer is:
houston
'nasa center' is the definition.
(location of NASA command center)
'hard to supplant working' is the wordplay.
'hard' becomes 'h'.
'to supplant' becomes 'oust' (I've seen this before).
'working' becomes 'on'.
'h'+'oust'+'on'='HOUSTON'
(Other definitions for houston that I've seen before include "American city" , "Texas city (with a problem?)" , "Texas oil port" , "American place" , "Texan port" .)