What machine gunners do seconds after tanks (6)
I believe the answer is:
strafe
'what machine gunners do' is the definition.
(strafe means to attack from the air)
'seconds after tanks' is the wordplay.
'seconds' becomes 's'.
'tanks' indicates an anagram (tank can mean to fail or go wrong).
'after' with letters rearranged gives 'trafe'.
's'+'trafe'='STRAFE'
(Other definitions for strafe that I've seen before include "Pepper" , "Faster (anag.)" , "use machine-guns" , "Rake with machine-gun fire" , "Bombard" .)