Jumper may have this very small winning margin (1,4)
I believe the answer is:
v neck
'jumper may have this' is the definition.
(I know that a jumper might have a v-neck)
'very small winning margin' is the wordplay.
'very small' becomes 'v' (abbreviated or small form of 'very').
'winning margin' becomes 'neck' (I've seen this in another clue).
'v'+'neck'='V-NECK'
(Other definitions for v neck that I've seen before include "Shape of a pullover perhaps" , "design of pullover?" , "Top edge to slip, possibly" , "Sort of sweater" , "alternative to polo?" .)