Boulder won't stay firm apparently (4)
I believe the answer is:
rock
'boulder' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rock that I've seen before include "Sweet < music" , "solid?" , "Be unstable; lull (baby)" , "Seaside sweet" , "Stagger" .)