Mature East German blocks fencing thrust (7)
I believe the answer is:
riposte
'fencing thrust' is the definition.
(I've seen this in another clue)
'mature east german blocks' is the wordplay.
'mature' becomes 'ripe' (synonyms).
'east german' becomes 'ost' ('east' in German).
'blocks' is an insertion indicator (the inside letters form a 'blockage').
'ripe' placed around 'ost' is 'RIPOSTE'.
(Other definitions for riposte that I've seen before include "Quick sharp reply" , "Quick retort or return thrust in fencing" , "Fencing counterattack" , "Fencer's counterattack" , "Quick and witty reply" .)