Go off base following former policeman (7)
I believe the answer is:
explode
'go off' is the definition.
(I know that go off can be written as explode)
'base following former policeman' is the wordplay.
'base' becomes 'e' (mathematical symbol).
'following' says to put letters next to each other.
'former' becomes 'ex' (prefix meaning former or onetime).
'policeman' becomes 'plod' (UK informal term for a police officer).
'ex'+'plod'='explod'
'e' after 'explod' is 'EXPLODE'.
(Other definitions for explode that I've seen before include "Burst violently" , "Pop" , "Go off with a bang" , "Do expel (anag)" , "Burst outward" .)