Lowlife types coming up for A-lister (4)
I believe the answer is:
star
'a-lister' is the definition.
(both can mean someone famous)
'lowlife types coming up' is the wordplay.
'lowlife' becomes 'rat' (synonyms).
'types coming' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'rat'+'s'='rats'
'rats' in reverse letter order is 'STAR'.
'for' is the link.
Can you help me to learn more?
(Other definitions for star that I've seen before include "Celebrated actor - heavenly body!" , "Public figure" , "virtuoso" , "For example, Marilyn Monroe" , "Sign of a leading performer" .)