In helmet, rookie underground (5)
I believe the answer is:
metro
'underground' is the definition.
(I know that underground can be written as metro)
'in helmet rookie' is the wordplay.
'in' says the answer is hidden in the clue.
'METRO' is hidden within 'helmet rookie'.
(Other definitions for metro that I've seen before include "Paris underground rail system" , "Paris subway" , "French underground" , "Parisian underground railway" , "Urban railway system" .)