Male model uncovered rising bias (8)
I believe the answer is:
nepotism
'bias' is the definition.
(I know that nepotism is a type of bias)
'male model uncovered rising' is the wordplay.
'male' becomes 'm' (common abbreviation).
'model' becomes 'sit' (e.g. for a painting).
'uncovered' becomes 'open' (I've seen this before).
'rising' says the letters should be written backwards (going up in a down clue).
'm'+'sit'+'open'='msitopen'
'msitopen' backwards is 'NEPOTISM'.
(Other definitions for nepotism that I've seen before include "Favouritism shown to relatives by those in power" , "Jobs for the boys" , "The practice of favouring one's relatives in making official appointments" , "Favouritism shown in hiring relatives" , "Favouritism shown to friends and family" .)