Sign of upcoming Labour power? Being led by tramp! (4)
I believe the answer is:
bump
'sign of upcoming labour' is the definition.
(as in an expectant mother going into labour)
'power? being led by tramp' is the wordplay.
'power?' becomes 'p' (symbol in physics).
'being led by' means one lot of letters go next to another.
'tramp' becomes 'bum' (both can mean a homeless person).
'p' after 'bum' is 'BUMP'.
(Other definitions for bump that I've seen before include "Jolting movement or skin swelling" , "Bash into" , "Small swelling" , "Jostle" , "Knock - collide" .)