In the grid a letter is added each level as you descend to make a new word according to the clues provided and then a letter is subtracted from each level in the bottom half of the grid in much the same way.

CLUES IN ORDER :-

Centre of Pressure

Top or school headgear

Speed or step

Room or The Final Frontier

Unit of astronomical length based on the distance from Earth at which stellar parallax is 1 sec of arc

Plural of the several separated coloured bands in lightbeams

Exercise until you get it right

Minute portion of matter or uninflected word

Exact copies or facsimiles

Particular

Join together

Appliances for holding or joining things together

Lose footing or make a mistake

Mouth or rim

Old vinyl record