Extremely alert after grotto warning (6)
I believe the answer is:
caveat
'warning' is the definition.
(caveat is a kind of warning)
'extremely alert after grotto' is the wordplay.
'extremely' means to remove the middle letters.
'after' is a charade indicator (letters next to each other).
'grotto' becomes 'cave' (I've seen this before).
'alert' with its centre removed is 'at'.
'at' put after 'cave' is 'CAVEAT'.
(Other definitions for caveat that I've seen before include "At cave (anag.)" , "Vacate (anag) - warning" , "'Caution, warning (6)'" , "Warning (of conditions)" , "'Warning, proviso (6)'" .)