Solver rapidly worked out “senior cabinet minister” (4,5,4)
I believe the answer is:
lord privy seal
'senior cabinet minister' is the definition.
(I've seen this before)
'solver rapidly worked out' is the wordplay.
'worked out' is an anagram indicator.
'solver'+'rapidly'='solverrapidly'
'solverrapidly' with letters rearranged gives 'LORD PRIVY SEAL'.
(Other definitions for lord privy seal that I've seen before include "Top cabinet minister" , "UK cabinet minister without specific departmental duties" , "Senior minister without portfolio, usually Leader of the House of Commons or Lords" , "minister's office" , "high official" .)