Officer vain at heart gave up backing a politician (4-2-4)
I believe the answer is:
aide-de-camp
'officer' is the definition.
(I've seen this before)
'vain at heart gave up backing a politician' is the wordplay.
'at heart' means to look at the middle letters.
'gave up' becomes 'ceded' ('cede' can be a synonym of 'give up').
'backing' says the letters should be written in reverse.
'politician' becomes 'MP' (Member of Parliament).
The middle letters of 'vain' are 'ai'.
'ceded' written backwards gives 'dedec'.
'ai'+'dedec'+'a'+'mp'='AIDE-DE-CAMP'
(Other definitions for aide-de-camp that I've seen before include "Military assistant" , "Military adviser" , "Senior officer's assistant" .)