Reveal first of MPs stopping Sunak, possibly (6)
I believe the answer is:
unmask
'reveal' is the definition.
(I've seen this in another clue)
'first of mps stopping sunak possibly' is the wordplay.
'first of mps' becomes 'm' (1st letter of 'mps').
'stopping' means one lot of letters goes inside another (inserted letters stop or plug a gap).
'possibly' indicates an anagram.
'sunak' is an anagram of 'unask'.
'm' placed inside 'unask' is 'UNMASK'.
(Other definitions for unmask that I've seen before include "Reveal, expose" , "Identify (someone hidden)" , "Expose, remove cover" , "Disclose (the hidden nature of)" , "Reveal true identity of" .)