Go out! Grotto is collapsing! Run! (4)
I believe the answer is:
trot
'run' is the definition.
(trotting is a kind of running)
'go out grotto is collapsing' is the wordplay.
'out' is a deletion indicator.
'is collapsing' is an anagram indicator.
'grotto' with 'go' taken out is 'rtto'.
'rtto' with letters rearranged gives 'TROT'.
(Other definitions for trot that I've seen before include "Left-winger - move at a fair speed" , "Old communist" , "Follower of revolutionary" , "Moderate running pace" , "A slowish run, often by horse" .)