Preserve agreement to put in German marks (3)
I believe the answer is:
jam
'preserve' is the definition.
(I know that jam is a type of preserve)
'agreement to put in german marks' is the wordplay.
'agreement to put in german' becomes 'ja' ('ja' is 'yes' translated into German).
'marks' becomes 'M' (former German currency).
'ja'+'m'='JAM'
(Other definitions for jam that I've seen before include "Sweet stuff for bread - stuff it in" , "Spread; improvise (jazz)" , "awful traffic" , "Preserve; pack tightly" , "Conserve" .)