Work space vacated round Post Office counter (6)
I believe the answer is:
oppose
'counter' is the definition.
(thesaurus)
'work space vacated round post office' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'vacated' suggests removing the centre.
'round' means one lot of letters goes inside another.
'post office' becomes 'po'.
'space' with its centre taken out is 'se'.
'op'+'se'='opse'
'opse' going around 'po' is 'OPPOSE'.
(Other definitions for oppose that I've seen before include "Actively resist" , "Resist, go against" , "Argue against" , "Resist, contend with" , "Refute - confront" .)