Jumper of sorts sprinted, covering a good length (6)
I believe the answer is:
raglan
'jumper of sorts' is the definition.
(type of garment)
'sprinted covering a good length' is the wordplay.
'sprinted' becomes 'ran' (I've seen this in another clue).
'covering' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'length' becomes 'l' (abbreviation in geometry).
'a'+'g'+'l'='agl'
'ran' placed around 'agl' is 'RAGLAN'.
(Other definitions for raglan that I've seen before include "Style of sleeve" , "coat with unique sleeves" , "A lord, a type of sleeve and a Dublin road made famous by Patrick Kavanagh" , "overcoat without shoulder seams" , "old military leader" .)