Staunch, upstanding police force succeeded (4)
I believe the answer is:
stem
'staunch' is the definition.
(I've seen this before)
'upstanding police force succeeded' is the wordplay.
'upstanding' says the letters should be written backwards.
'police force' becomes 'met' (short for Metropolitan Police Service).
'succeeded' becomes 's' (genealogical abbreviation).
'met'+'s'='mets'
'mets' back-to-front is 'STEM'.
(Other definitions for stem that I've seen before include "Slender vertical part, glass or plant" , "Shoot; arrest" , "Peduncle" , "Tube" , "supporter of tobacco?" .)