Sharp pain from cracked back (4)
I believe the answer is:
stab
'sharp pain' is the definition.
(I've seen this in another clue)
'cracked back' is the wordplay.
'cracked' becomes 'bats' (synonyms).
'back' says the letters should be written in reverse.
'bats' written backwards gives 'STAB'.
'from' acts as a link.
(Other definitions for stab that I've seen before include "wound as Casca" , "Try - skewer" , "Injure with knife" , "Plunge knife into" , "Sudden sharp feeling" .)