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 tell whether this definition defines the answer.
'one on the slide' is the wordplay.
'one' becomes 'er' (I can't justify 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' acts as a 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" .)