Male model, topless, backing undue favouritism (8)
I believe the answer is:
nepotism
'undue favouritism' is the definition.
'nepotism' can be an answer for 'favouritism' (nepotism is a kind of favouritism). I am unsure of the 'undue' bit.
'male model topless backing' is the wordplay.
'male' becomes 'm' (common abbreviation).
'model' becomes 'sit' (e.g. for a painting).
'topless' becomes 'open' (opened up and without a top).
'backing' says the letters should be written in reverse.
'm'+'sit'+'open'='msitopen'
'msitopen' back-to-front is 'NEPOTISM'.
Can you help me to learn more?
(Other definitions for nepotism that I've seen before include "Pimentos (anag.)" , "Favouritism shown in hiring relatives" , "Pro-family bias" , "Showing favour" , "The practice of favouring one's relatives in making official appointments" .)