Gunners outline hectic competition (3,4)
I believe the answer is:
rat race
'hectic competition' is the definition.
(I've seen this before)
'gunners outline' is the wordplay.
'gunners' becomes 'ra' (Royal Artillery).
'outline' becomes 'trace' ('trace' can be a synonym of 'outline').
'ra'+'trace'='RAT RACE'
(Other definitions for rat race that I've seen before include "Competitive daily existence" , "life's futile struggle" , "keen contest" , "Competition for power" , "Struggle to survive" .)