Subject: Cannot use "diff" in a function assignment.
From: Richard Fateman
Date: Wed, 23 Nov 2011 15:05:25 -0800
Unless you are a good deal more sophisticated, simply do not use define.
Use assignment, for which Maxima uses a colon (:).
For example,
dm1 : at( diff(m(x),x), x=1)
although as you have defined m, diff(m(x),x) is 0.