Desertion from advanced position, simple losing head (8)
I believe the answer is:
apostasy
'desertion' is the definition.
(apostasy is a kind of desertion)
'advanced position simple losing head' is the wordplay.
'advanced' becomes 'a'.
'position' becomes 'post' (post is a kind of position**).
'simple' becomes 'easy' ('easy' is associated in meaning with 'simple').
'losing head' means to remove the first letter.
'easy' with its initial letter taken away is 'asy'.
'a'+'post'+'asy'='APOSTASY'
'from' is the link.
(Other definitions for apostasy that I've seen before include "Renunciation of one's beliefs or principles" , "Renunciation of religious or political beliefs" , "Ditching faith" , "Rejection of religion" , "defection" .)