Let loose under second officer (7)
I believe the answer is:
mountie
'officer' is the definition.
(Mountie is an example)
'let loose under second' is the wordplay.
'let loose' becomes 'untie' (I've seen this before**).
'under' means one lot of letters go next to another.
'second' becomes 'mo' (both are short periods of time).
'untie' after 'mo' is 'MOUNTIE'.
(Other definitions for mountie that I've seen before include "North American policeman" , "Canadian policeman" , "The lawman always gets his man!" , "One boasting of getting man" , "One policeman" .)