Unreasonable German with EU backing (5)
I believe the answer is:
undue
'unreasonable' is the definition.
(I know that unreasonable can be written as undue)
'german with eu backing' is the wordplay.
'german with' becomes 'und' (German for 'and' or 'with').
'backing' is a reversal indicator.
'eu' reversed gives 'ue'.
'und'+'ue'='UNDUE'
(Other definitions for undue that I've seen before include "Extreme" , "Excessive - unwarranted" , "too much" , "Excessive amount" , "Excessive, unreasonable" .)