Nasty European immersed in ten gallons, perhaps (7)
I believe the answer is:
hateful
'nasty' is the definition.
(similar in meaning)
'european immersed in ten gallons perhaps' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'immersed in' is an insertion indicator.
'ten gallons perhaps' becomes 'hatful' (ten-gallon is a type of hat).
'e' put into 'hatful' is 'HATEFUL'.
(Other definitions for hateful that I've seen before include "Abhorrent" , "Dreadful" , "Detestable" , "repellent" , "Loathsome" .)