Grand commander getting slightly mad (4)
I believe the answer is:
gaga
'slightly mad' is the definition.
(I've seen this before)
'grand commander getting' is the wordplay.
'grand' becomes 'g' ('G' can be a synonym of 'grand').
'commander' becomes 'aga' (I've seen this before**).
'getting' means one lot of letters go next to another.
'g'+'aga'='GAGA'
(Other definitions for gaga that I've seen before include "Lady < > in a senile" , "US singer Lady ___" , "Slangily and repetitively crazy" , "Queen's radio recording" , "Slangily crazy or senile" .)