Sorry folk, I may well be losing the run of myself as in the cold light of day it all seems a bit tenuous, and not spot on.
The second meaning of ply is to work steadily/diligently, and the meaning of the common name in the first appendix in Chambers pre 12th edition - sounds the same, just add an "a" - is not too dissimilar.
Excellent crossword as usual from Schadenfreude.
Isn't it just that you have to remove the letter 'a' from the common theme word in the play title and, therefore, from the word play as well? Thanks for the correspondence which has helped me complete it today.