Shell explodes twice, bringing police (4)
I believe the answer is:
plod
'police' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how one could define the other.
'shell explodes twice' is the wordplay.
'PLOD' is hidden in amongst the letters.
However, I am unsure how this is indicated.
'bringing' acts as a link.
(Other definitions for plod that I've seen before include "Trudge wearily" , "grind away" , "Copper" , "Work (or walk) laboriously" , "Fuzzy type" .)