Losing no time, pop back in to free (5)
I believe the answer is:
rapid
'losing no time' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'pop back in to free' is the wordplay.
'pop' becomes 'pa' (both can mean father).
'back' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'to free' becomes 'rid' (to be free of something is to be rid of it).
'pa' reversed gives 'ap'.
'ap' put within 'rid' is 'RAPID'.
Can you help me to learn more?
(Other definitions for rapid that I've seen before include "Speedy, quick" , "Speedy (5)" , "Fast, quick" , "Express" , "Waterway where the current runs very fast" .)