Almost sure to crack acrobatic feat (10)
I believe the answer is:
somersault
'acrobatic feat' is the definition.
(I know that somersault is a type of tumble)
'almost sure to crack' is the wordplay.
'to crack' indicates an anagram (I've seen 'cracked' mean this).
'almost'+'sure'='almostsure'
'almostsure' with letters rearranged gives 'SOMERSAULT'.
(Other definitions for somersault that I've seen before include "Feat of acrobatics" , "Turn head over heels" , "Move acrobatically" , "policy switch" , "Acrobatic movement" .)