Target of nepotism? Measures one’s time in office: short! (10)
I believe the answer is:
stepsister
'target of nepotism?' is the definition.
(one's stepsister might benefit from nepotism)
'measures one's time in office short' is the wordplay.
'measures' becomes 'steps' ('step' can be a synonym of 'measure').
'one's' becomes 'I's' (Roman numeral).
'time in office' becomes 'term' (eg a presidential term).
'short' means to remove the last letter.
'term' with its final letter taken off is 'ter'.
'steps'+'is'+'ter'='STEPSISTER'
(Other definitions for stepsister that I've seen before include "one who hated Cinderella" , "Relation gained through a second marriage" , "member of the family" , "relative in law" .)