An offer in German is enduring (7)
I believe the answer is:
abiding
'is enduring' is the definition.
(synonyms)
'an offer in german' is the wordplay.
'an' becomes 'a'.
'offer' becomes 'bid' (synonyms).
'german' becomes 'g' (abbreviation).
'a'+'bid'+'in'+'g'='ABIDING'
(Other definitions for abiding that I've seen before include "Permanent" , "standing" , "Unceasing or putting up with" , "Persisting" , "Durable, lasting" .)