Queen's 'Under Pressure' with huge exhibitionist (5)
I believe the answer is:
poser
'exhibitionist' is the definition.
(poser is a kind of exhibitionist)
'queen's under pressure with huge' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina**).
'under' means one lot of letters go next to another.
'pressure' becomes 'p' (abbreviation).
'with' means one lot of letters go next to another.
'huge' becomes 'os' (abbreviation for outsize).
'p'+'os'='pos'
'er' after 'pos' is 'POSER'.
(Other definitions for poser that I've seen before include "Tricky clue" , "subject for an artist?" , "Exhibitionist" , "Tricky problem" , "Difficult problem" .)