Senile German commander (4)
I believe the answer is:
gaga
'senile' is the definition.
('gaga' can be a synonym of 'senile')
'german commander' is the wordplay.
'german' becomes 'g' (abbreviation).
'commander' becomes 'aga' (I've seen this before**).
'g'+'aga'='GAGA'
(Other definitions for gaga that I've seen before include "Slangily crazy or senile" , "bananas" , "Lady < > in a senile" , "Slightly mad (colloq.)" , "Barmy joke, third in cracker" .)