Share ground not air (6)
I believe the answer is:
ration
'share' is the definition.
(a ration is a share of a limited resource)
'ground not air' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'not'+'air'='notair'
'notair' with letters rearranged gives 'RATION'.
(Other definitions for ration that I've seen before include "Restricted share" , "Dole out in measured quantities, as in time of shortage" , "Limited allocation of food etc in wartime" , "Apportion" , "Fixed restricted allowance" .)