East German in mature comeback (7)
I believe the answer is:
riposte
'comeback' is the definition.
(I know that comeback can be written as riposte)
'east german in mature' is the wordplay.
'east german' becomes 'ost' ('east' in German).
'in' means one lot of letters goes inside another.
'mature' becomes 'ripe' (synonyms).
'ost' going within 'ripe' is 'RIPOSTE'.
(Other definitions for riposte that I've seen before include "Fencing counterattack" , "Quick sharp reply" , "Quick retort or return hit in fencing" , "Counter-thrust" , "Snappy comeback" .)