Initially statesmanlike, it's said Starmer is one on the slide (5)
I believe the answer is:
skier
'initially statesmanlike it's said starmer' is the definition.
I can't judge whether this defines the answer.
'one on the slide' is the wordplay.
'one' becomes 'er' (I can't explain this - if you can you should believe this answer much more).
'on' is a charade indicator (letters next to each other).
'the slide' becomes 'ski' (skis slide over snow).
'er' put after 'ski' is 'SKIER'.
'is' is the link.
Can you help me to learn more?
(Other definitions for skier that I've seen before include "sporting figure" , "One up mountain, perhaps" , "Sportsman who glides over snow" , "he's possibly going downhill quickly" , "Winter sportsperson" .)