Petrify East German finally at the borders (6)
I believe the answer is:
ossify
'petrify' is the definition.
(I know that petrify can be written as ossify)
'east german finally at the borders' is the wordplay.
'east german' becomes 'Ossi' (informal term for a citizen of East Germany).
'at the borders' means to remove the middle letters (the letters at the borders of the word).
'finally' with its centre removed is 'fy'.
'ossi'+'fy'='OSSIFY'
(Other definitions for ossify that I've seen before include "Make rigid" , "Change into bone, or become unprogressive" , "Get hard" , "Turn to bone with age" , "Solidify; turn into bone" .)