Trouble very much engulfs Republican rating (6)
I believe the answer is:
sailor
'rating' is the definition.
(rating can mean a navy sailor)
'trouble very much engulfs republican' is the wordplay.
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble').
'very much' becomes 'so'.
'engulfs' is an insertion indicator.
'republican' becomes 'r' (abbreviation).
'ail' put into 'so' is 'sailo'.
'sailo'+'r'='SAILOR'
(Other definitions for sailor that I've seen before include "Old salt" , "One in the main" , "Sinbad, for example" , "Seaman" , "Sinbad, maybe" .)