Thanks to Tramp for a real workout.
I found this rather a beast and am proud to have filled the grid. Many difficult parsings.
FOI 13.
The combination of 14d and 16 held me back forever, but 16 (tortured parsing) was LOI by a good margin.
I actually rate 21a as COD for sheer trickiness and forehead slap after PDM. 16 also quite good, but was so painful to solve that I can't rate it on top. Quite a few other candidates, incl. several short word solutions.
Still not sure of parsing of 11 and 14a.