Main objection concerns easily acquired nausea primarily (5)
I believe the answer is:
ocean
'main' is the definition.
('ocean' can be a synonym of 'main')
'objection concerns easily acquired nausea primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'objection concerns easily acquired nausea' is 'OCEAN'.
(Other definitions for ocean that I've seen before include "Large amount of water and" , "Great sea" , "Large expanse of water" , "Indian or Arctic, say" , "A lot of water!" .)