Policeman's bad run following diamonds (7)
I believe the answer is:
officer
'policeman's' is the definition.
('officer' can be a synonym of 'policeman')
'bad run following diamonds' is the wordplay.
'bad' becomes 'off' (I've seen this before).
'run' becomes 'r' (cricket abbreviation).
'following' means one lot of letters go next to another.
'diamonds' becomes 'ice' (I've seen this before).
'r' put after 'ice' is 'icer'.
'off'+'icer'='OFFICER'
(Other definitions for officer that I've seen before include "Holder of a commission in the armed forces" , "Police(wo)man" , "Holder of responsible position" , "who has a public position" , "Detective" .)