Three months you ultimately spent guarding Los Angeles prison (7)
I believe the answer is:
slammer
'prison' is the definition.
(I know that slammer is a type of prison)
'three months you ultimately spent guarding los angeles' is the wordplay.
I cannot really understand how this works, but
'months' could be 'mm' (an abbreviation for month written twice) and 'mm' is present in the answer.
'los angeles' could be 'la' (abbreviation) and 'la' is located in the answer.
The remaining letters 'ser' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for slammer that I've seen before include "Stir" , "He noisily leaves the room" , "Prison (slang)" , "one leaping into crowd" , "Prison - critic?" .)