Letter supporting Germany is lacking consideration (10)
I believe the answer is:
dismissive
'lacking consideration' is the definition.
'letter supporting germany is' is the wordplay.
'letter' becomes 'missive' ('missive' can be a synonym of 'letter'**).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'germany' becomes 'd' (international vehicle registration code).
'd'+'is'='dis'
'missive' put after 'dis' is 'DISMISSIVE'.
(Other definitions for dismissive that I've seen before include "Disdainful" , "sneering?" , "contemptuous" , "rebuffing me?" .)