Parliamentary official missing first encouragement to cheer (3)
I believe the answer is:
hip
'encouragement to cheer' is the definition.
(hip hip hooray!)
'parliamentary official missing first' is the wordplay.
'parliamentary official' becomes 'whip' (whips enforce party discipline in parliament).
'missing first' means to remove the first letter.
'whip' with its first letter removed is 'HIP'.
(Other definitions for hip that I've seen before include "Fruit of a rose plant" , "Pelvic joint" , "Pelvis" , "Trendy bodily joint?" , "Haunch" .)