One who’s “dreaded” retiring, beats a retreat to downsize (5)
I believe the answer is:
rasta
'one who's dreaded' is the definition.
(a Rastafarian might have dreadlocks)
'retiring beats a retreat to downsize' is the wordplay.
'retiring' shows that the letters should be reversed in order.
'to downsize' says the answer is hidden in the clue (part of the clue must downsize or become shorter).
'atsar' is hidden within 'beats a retreat'.
'atsar' written backwards gives 'RASTA'.
(Other definitions for rasta that I've seen before include "Member of religious group" , "religious follower" , "A certain believer" , "Jamaican sect member" , "Jamaican cultist" .)