Innovative engineers into clay-firing (14)
I believe the answer is:
groundbreaking
'innovative' is the definition.
(I know that innovative can be written as groundbreaking)
'engineers into clay-firing' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'into' indicates putting letters inside.
'clay' becomes 'ground' (the ground might be made of clay).
'firing' becomes 'baking' (as in baking or firing pottery).
'ground'+'baking'='groundbaking'
're' inserted within 'groundbaking' is 'GROUNDBREAKING'.
(Other definitions for groundbreaking that I've seen before include "Pioneering" , "Daring" .)