Long right jabs this close to heavyweight (6)
I believe the answer is:
thirst
'long' is the definition.
(thirst can mean to long or yearn for something)
'right jabs this close to heavyweight' is the wordplay.
'right' becomes 'r' (common abbreviation).
'jabs' indicates putting letters inside (some letters jab into others).
'close to' indicates one should take the final letters (the close/ending of).
The last letter of 'heavyweight' is 't'.
'r' put within 'this' is 'thirs'.
'thirs'+'t'='THIRST'
(Other definitions for thirst that I've seen before include "Want of drink" , "Craving for liquid" , "Drink to slake it" , "lack of water" , "Strong desire from dehydration?" .)