\[ \boxed { {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) -a \left ( 1+ \left ( \tan \left ( y \left ( x \right ) \right ) \right ) ^{2} \right ) +\tan \left ( y \left ( x \right ) \right ) \tan \left ( x \right ) =0} \]
Mathematica: cpu = 49.920839 (sec), leaf count = 27 \[ \text {DSolve}\left [-a \left (\tan ^2(y(x))+1\right )+y'(x)+\tan (x) \tan (y(x))=0,y(x),x\right ] \]
Maple: cpu = 8.471 (sec), leaf count = 0 \[ \text {could not solve} \]