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' indicates the answer is hidden within the clue (a series of letters from the clue).
'in reverse' is a reversal indicator.
'tsegral' can be found hidden inside 'stunts e g rally driving'.
'tsegral' written backwards gives 'LARGEST'.
(Other definitions for largest that I've seen before include "Extremely generous" , "Superlatively extensive" , "Most substantial" , "Biggest" , "Of greatest magnitude" .)