Senior cabinet minister provides rally in reshuffle (4,5,4)
I believe the answer is:
lord privy seal
'senior cabinet minister' is the definition.
(I've seen this before)
'provides rally in reshuffle' is the wordplay.
'in reshuffle' indicates an anagram (letters get shuffled around).
'provides'+'rally'='providesrally'
'providesrally' is an anagram of 'LORD PRIVY SEAL'.
(Other definitions for lord privy seal that I've seen before include "Top cabinet minister" , "government office" , "high official" , "minister's office" , "Senior minister without portfolio, usually Leader of the House of Commons or Lords" .)