Good making up numbers for basic training (9)
I believe the answer is:
grounding
'basic training' is the definition.
(I've seen this before)
'good making up numbers' is the wordplay.
'good' becomes 'g' (abbreviation).
'making up numbers' becomes 'rounding'.
'g'+'rounding'='GROUNDING'
'for' is the link.
(Other definitions for grounding that I've seen before include "keeping home" , "Rudimentary knowledge" , "hampering progress of high flier?" , "Punishment by parent" , "Foundation of basic knowledge" .)