Solution for stiff locks (7)
I believe the answer is:
lacquer
'solution' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for lacquer that I've seen before include "Hard shiny covering on wood and metal" , "protective coat" , "Hard coating for wood" , "Shiny varnish" , "Needing strong finish" .)