The sort of income you ultimately require to retain a service (8)
I believe the answer is:
unearned
'the sort of income' is the definition.
'you ultimately require to retain a service' is the wordplay.
'ultimately' indicates one should take the final letters.
'require' becomes 'need' ('need' can be a synonym of 'require'**).
'to retain' indicates putting letters inside.
'service' becomes 'rn' (abbreviation for Royal Navy).
The last letter of 'you' is 'u'.
'a'+'rn'='arn'
'need' placed around 'arn' is 'nearned'.
'u'+'nearned'='UNEARNED'
(Other definitions for unearned that I've seen before include "Unmerited" , "Not gained by labour" , "Not merited by work, like some income" , "Not deserved" .)