Boy, turning scarlet close to girls, runs (7)
I believe the answer is:
ladders
'runs' is the definition.
('ladder' can be a synonym of 'run')
'boy turning scarlet close to girls' is the wordplay.
'boy' becomes 'lad' (lad is a kind of boy**).
'turning' says the letters should be written in reverse.
'scarlet' becomes 'red' ('red' can be a synonym of 'scarlet').
'close to' suggests the final letters (the close/ending of).
The final letter of 'girls' is 's'.
'red' reversed gives 'der'.
'lad'+'der'+'s'='LADDERS'
(Other definitions for ladders that I've seen before include "Rows of rungs" , "Runs in hosiery" , "Climbing devices" , "Raddles (anag.)" , "Sets of steps" .)