Most prodigious series of stunts e.g. rally-driving in reverse (7)
I believe the answer is:
largest
'most prodigious' is the definition.
('large' can be similar in meaning to 'prodigious')
'series of stunts e g rally-driving in reverse' is the wordplay.
'series of' says the answer is hidden in the clue (a series of letters from the clue).
'in reverse' shows that the letters should be reversed in order.
'tsegral' can be found hidden inside 'stunts e g rally driving'.
'tsegral' in reverse letter order is 'LARGEST'.
(Other definitions for largest that I've seen before include "Extremely generous" , "Superlatively extensive" , "Most substantial" , "Biggest" , "Of greatest magnitude" .)