Firm that's erected circus tent employs well-armed individuals (9)
I believe the answer is:
octopuses
'well-armed individuals' is the definition.
(creatures with eight tentacles or 'arms')
'firm that's erected circus tent employs' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'that's erected' says the letters should be written backwards.
'circus tent' becomes 'top' (synonyms).
'employs' becomes 'uses' ('use' can be a synonym of 'employ').
'co' backwards is 'oc'.
'oc'+'top'+'uses'='OCTOPUSES'
(Other definitions for octopuses that I've seen before include "Multi-armed sea creatures" , "influential organisations" , "With eight long tentacles, they live in the deep" , "Cephalopod molluscs" , "Eight-armed cephalopods" .)