Recruit showing some serious progress (5)
I believe the answer is:
sprog
'recruit' is the definition.
(I know that sprog is a type of recruit)
'some serious progress' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'SPROG' is hidden in the letters of 'serious progress'.
'showing' is the link.
(Other definitions for sprog that I've seen before include "Infant" , "Kid" , "Child, colloquially" , "Child; recruit (mil. slang)" , "Baby (slang)" .)