Political assistant has quiet time seizing power (6)
I believe the answer is:
'political assistant' is the definition.
(someone who prepares for a political summit)
'quiet time seizing power' is the wordplay.
'quiet' becomes 'sh' (both can be said to tell someone to be quiet).
'time' becomes 'era' (era is a kind of time).
'seizing' means one lot of letters goes inside another.
'power' becomes 'p'.
'era' placed around 'p' is 'erpa'.
'has' acts as a link.
(Other definitions for sherpa that I've seen before include "Himalayan mountain guide", "Himalayan - phrase (anag)", "Nepalese mountaineer", "High-up employee", "Experienced climber".)