Bring in worker, that's essential (9)
I believe the answer is:
important
'that's essential' is the definition.
(similar in meaning)
'bring in worker' is the wordplay.
'bring in' becomes 'import' (bring in to a country).
'worker' becomes 'ant' (worker is a type of ant).
'import'+'ant'='IMPORTANT'
(Other definitions for important that I've seen before include "Eminent" , "Noteworthy, significant" , "major" , "A priority" , "Significant, valuable" .)