After popular late ruler, note a disinclination to act (7)
I believe the answer is:
inertia
'disinclination to act' is the definition.
(I've seen this before)
'after popular late ruler note a' is the wordplay.
I cannot really understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is found in the answer.
'ruler' could be 'er' (short for Elizabeth Regina ie Queen Elizabeth) and 'er' is found within the answer.
'note' could be 'ti' (musical note, as in do re mi so fa la ti do) and 'ti' is located in the answer.
'a' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for inertia that I've seen before include "A disposition to remain inactive" , "Lack of activity" , "I retain, being without energy" , "Tendency not to act" , "Unwillingness to move" .)