Military post is set up in troubled period (8)

'military post' is the definition.
(type of military settlement)

'is set up in troubled period' is the wordplay.
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'in' indicates putting letters inside.
'troubled' indicates an anagram.
'is' back-to-front is 'si'.
'period' is an anagram of 'predio'.
'si' placed within 'predio' is 'PRESIDIO'.