Stuck fast (7)
I believe the answer is:
adhered
'stuck fast' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for adhered that I've seen before include "Stuck (to something)" , "Stuck; glued" , "Stuck to, remained loyal" , "Showed sticking power" , "Stuck - redhead (anag)" .)