Terribly vain son retains one occupation (8)
I believe the answer is:
invasion
'occupation' is the definition.
(I've seen this before)
'terribly vain son retains one' is the wordplay.
'terribly' indicates an anagram.
'retains' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'son' going around 'i' is 'sion'.
'vain' is an anagram of 'inva'.
'inva'+'sion'='INVASION'
(Other definitions for invasion that I've seen before include "Incursive attack" , "Encroachment, intrusion" , "Occupation" , "Unwelcome encroachment on another country" , "Incursion, entry for conquest" .)