King George IV clings to throne, ultimately in era of trouble (8)
I believe the answer is:
aggrieve
'trouble' is the definition.
(I've seen this before)
'king george iv clings to throne ultimately in era' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'clings to' indicates putting letters inside.
'ultimately' says to take the final letters.
'in' is an insertion indicator.
'era' becomes 'age' (I've seen this before).
The last letter of 'throne' is 'e'.
'iv' placed around 'e' is 'iev'.
'gr'+'iev'='griev'
'griev' going into 'age' is 'AGGRIEVE'.
'of' is the link.
(Other definitions for aggrieve that I've seen before include "Pain or injure" , "Trouble" , "Make resentful" , "Cause pain to; distress" , "upset" .)