Detectives flank nearest unoccupied protestor (9)
I believe the answer is:
dissident
'protestor' is the definition.
(thesaurus)
'detectives flank nearest unoccupied' is the wordplay.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'flank' becomes 'side' (I've seen this before).
'unoccupied' says to hollow out the word (remove centre letters) (remove the inside letters which occupy the word).
'nearest' with its middle removed is 'nt'.
'dis'+'side'+'nt'='DISSIDENT'
(Other definitions for dissident that I've seen before include "Opposer of officialdom" , "One opposed to official policy" , "Protester" , "Political nonconformist" , "Person opposing official policy" .)