As a time-saver, employing retro prefix (4)
I believe the answer is:
semi
'prefix' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'as a time-saver employing retro' is the wordplay.
'employing' indicates a hidden word (the answer is used or employed by the clue).
'retro' is a reversal indicator (retro is Latin for 'backwards').
'imes' is hidden in the letters of 'as time saver'.
'imes' reversed gives 'SEMI'.
Can you help me to learn more?
(Other definitions for semi that I've seen before include "Common type of house" , "match, the last but one?" , "The last but one round" , "One house of pair" , "important match" .)