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 am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'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" .)