Air-traffic control is unsuccessful at keeping fourth in proximity (2,5,8)
I believe the answer is:
at close quarters
'in proximity' is the definition.
'air-traffic control is unsuccessful at keeping fourth' is the wordplay.
'air traffic control' becomes 'atc'.
'is unsuccessful at' becomes 'loses' (to lose at something is to be unsuccessful at it).
'keeping' indicates putting letters inside.
'fourth' becomes 'quarter' (synonyms).
'loses' placed around 'quarter' is 'losequarters'.
'atc'+'losequarters'='AT CLOSE QUARTERS'
(Other definitions for at close quarters that I've seen before include "nearby" , "very near..." .)