Self-possession of prime minister, concealing hard-on (6)
I believe the answer is:
phlegm
'self-possession' is the definition.
(I've seen this before)
'prime minister concealing hard-on' is the wordplay.
'prime minister' becomes 'PM' (abbreviation).
'concealing' means one lot of letters goes inside another.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'on' becomes 'leg' (cricket term, fielding positions).
'h'+'leg'='hleg'
'pm' enclosing 'hleg' is 'PHLEGM'.
'of' acts as a link.
(Other definitions for phlegm that I've seen before include "Imperturbable stolidity" , "Thick mucous in the throat and bronchial passages" , "Mucous secretion" , "Mucus - self-possession" , "Calmness of temperament" .)